Makrut Lime: Essential Oil, Photosensitivity and Safety

This is the most practically important page in this set, and the only one where the evidence base is clinical rather than laboratory. Here is the single sentence it exists to deliver: citrus peel oils can react with sunlight and blister your skin, and the reaction is delayed, so you will not know it is happening until many hours later. If you are planning to make a hair oil, a scalp treatment, a body oil or a home insect repellent from makrut lime (still widely sold as kaffir lime), read this before you mix anything.

The mechanism has a name — phytophotodermatitis — and a nickname that tells you how ordinary it is: “margarita burn.” Bartenders who cut limes all evening and then work in sunlight get it on their hands. Holidaymakers who squeeze citrus at a beach bar get it in odd streaks and drips. The people most at risk from makrut lime specifically are the ones with the best intentions: home formulators making a leave-on skin or hair preparation from citrus peel oil.

The rest of this page covers the chemistry, the timeline of the reaction, the single most useful distinction in citrus-oil safety (steam-distilled versus cold-pressed), practical rules, dilution and patch testing, contact allergy as a separate problem, and the categorical prohibitions on internal use.

Read This Before You Make Anything
Furocoumarins: What They Are
How Phytophotodermatitis Happens
“Margarita Burn”: the Everyday Version
Steam-Distilled Versus Cold-Pressed
Leaf Oil Versus Peel Oil
Practical Rules for Skin and Hair
Dilution and Patch Testing
Contact Allergy and Oxidation
Never Internal
Pregnancy, Breastfeeding and Children
Furocoumarins and Medication
Buying, Labelling and Storing
If a Reaction Happens
Key Research Papers
Connections

Read This Before You Make Anything

The whole page compressed, for anyone who reads one section:

Cold-pressed citrus peel oil plus ultraviolet light equals a burn. Not an allergy, not an irritation — a chemical photoreaction that produces redness and blistering, followed by pigmentation that can last months.
The reaction is delayed. Nothing happens while you are in the sun. Redness typically begins roughly 12 to 24 hours later and peaks around 48 to 72 hours. By the time you know, the damage is done.
Steam-distilled oils are typically much lower in the responsible compounds than cold-pressed ones, because furocoumarins are heavy and non-volatile and largely stay behind in the still. This is the single most useful thing to know when buying.
Keep expressed citrus peel oil off skin that will see sunlight for at least 12–18 hours. That includes tanning beds.
Leaf oil is a lower-risk choice than peel oil for a leave-on skin preparation, because furocoumarins are principally a peel phenomenon.
Never apply any essential oil undiluted, and never swallow it.
Eating makrut lime is not the issue. The leaf in your soup and the zest in your curry paste are not a phototoxicity risk. This page is about concentrated oil and about handling large quantities of peel.

Furocoumarins: What They Are

Furocoumarins — also written furanocoumarins, and the subgroup most relevant here is the psoralens — are a family of plant compounds built from a coumarin ring fused to a furan ring. They are produced by a number of plant families as chemical defences, and two families are notorious for them: the carrot family (Apiaceae — giant hogweed, cow parsley, celery, parsnip) and the citrus family (Rutaceae — every citrus, plus rue itself, which is where the family name comes from).

The individual compounds you will see named:

Bergapten (5-methoxypsoralen, 5-MOP) — the classic citrus phototoxin, named after bergamot, where it occurs at high levels. The bergapten-type compounds are the main reason citrus peel oils are phototoxic.
Bergamottin — a larger furocoumarin abundant in citrus peel, best known outside dermatology as a component of the grapefruit–drug interaction.
Xanthotoxin (8-methoxypsoralen, 8-MOP) — the psoralen used medically in PUVA phototherapy for psoriasis, which is the clearest possible proof that these molecules do what is claimed: dermatologists deliberately combine a psoralen with ultraviolet A light to produce a controlled skin effect.
Psoralen itself, oxypeucedanin, epoxybergamottin and a long tail of related structures.

Where they sit in the fruit. Furocoumarins are concentrated in the peel — in the flavedo, the coloured outer rind where the oil glands are. The juice contains far less, the leaves less again. This distribution is the reason nearly every practical rule on this page is a rule about peel oil.

An honest gap in the evidence. The furocoumarin content of Citrus hystrix peel specifically has not been well quantified in the published literature. Bergamot and expressed lime oil are the well-characterised offenders; makrut is under-studied. That leaves two ways to reason, and only one of them is safe: assume the species is an exception, or treat it like its relatives. Treat expressed makrut peel oil the way you would treat expressed lime oil. The cost of that assumption if wrong is a slightly over-cautious hair oil. The cost of the opposite assumption if wrong is a blistered face.

How Phytophotodermatitis Happens

This is a well-understood photochemical reaction, and knowing the steps makes the practical rules obvious.

Contact. A psoralen-containing substance reaches the skin — peel oil in a blend, juice from a squeezed fruit, sap from a cut stem, oil transferred from your fingers to your face.
Absorption. Psoralens are small and lipophilic. They penetrate the epidermis readily and reach cell nuclei, where they intercalate between the base pairs of DNA. At this stage nothing is happening and nothing can be felt.
Activation by UVA. Psoralens absorb ultraviolet A light, roughly 320–400 nm. UVA is the long-wavelength component of sunlight that passes through window glass and through cloud, and it is the dominant output of tanning beds. Being in the shade or behind glass is not reliable protection.
Cross-linking. The energised psoralen forms covalent adducts with pyrimidine bases and can cross-link the two DNA strands. Cross-linked DNA cannot be replicated or transcribed. The keratinocyte dies. Reactive oxygen species add further damage to membranes and proteins.
Inflammation — delayed. The cell death and cytokine release that follow take hours to produce a visible reaction. Erythema typically begins around 12 to 24 hours after exposure and peaks at roughly 48 to 72 hours. Severe cases blister, sometimes dramatically, in the exact shape of wherever the substance was — drips, handprints, streaks, the outline of a leaf.
Post-inflammatory hyperpigmentation. As the acute reaction settles, melanocytes respond and the affected skin darkens in the same pattern. This pigmentation commonly lasts weeks to many months, and it is the reason the reaction matters cosmetically long after the pain has gone. On darker skin it can be both more pronounced and more persistent.

The three features that make this reaction easy to misdiagnose: it is delayed, so people do not connect it to the exposure; it is phototoxic rather than allergic, meaning it can happen to anyone on first exposure with no prior sensitisation and does not require an immune memory; and its bizarre shapes — streaks, drips, sharp margins — look nothing like sunburn and are frequently mistaken for burns, infection or abuse. Tier for all of the above: established clinical dermatology, supported by a substantial literature of case reports, reviews and the entire practice of PUVA therapy.

“Margarita Burn”: the Everyday Version

The classic scenario appears every summer in dermatology clinics. Someone cuts and squeezes limes — making drinks at a barbecue, working a bar, preparing food at a beach — then spends time in the sun with the residue on their hands. A day later the backs of the hands and fingers are red, itching or burning, sometimes blistered, in streaks and patches that follow where the juice ran. The pigmentation that follows can last for months. It is variously called margarita burn, margarita dermatitis or lime disease, and it is textbook phytophotodermatitis.

Steam-Distilled Versus Cold-Pressed

If you remember one buying rule from this page, make it this one.

Citrus peel oil is produced two ways, and they yield chemically different products:

Cold-pressed / expressed. The rind is mechanically pressed or abraded and the oil is separated. Nothing is heated, nothing is fractionated, and everything that was in the oil glands ends up in the bottle — including the heavy, non-volatile furocoumarins. This is the standard method for citrus peel oils because it gives the freshest, truest aroma. It is also the reason expressed citrus oils are the phototoxic ones.
Steam-distilled. Steam is passed through the plant material and the volatile compounds carry over with it and are condensed. Furocoumarins are comparatively large, heavy and non-volatile, so they largely do not make the journey and remain in the still residue. Steam-distilled citrus oils are therefore typically far lower in furocoumarins — often to the point of being treated as non-phototoxic. The trade-off is aroma: distilled citrus oil smells flatter and less vivid than expressed.
Furocoumarin-free (FCF) oils. A third category, produced by further processing — distillation or chromatographic removal — specifically to strip the phototoxic fraction. Bergamot FCF is the well-known example. If you see “FCF” or “bergapten-free” on a label, that is what it means.

Three practical consequences:

Read the extraction method on the label. A reputable supplier states it. If the label does not say, ask; if the supplier cannot answer, assume expressed and treat the oil as phototoxic.
“Cold-pressed” sounds gentler and is riskier here. This is genuinely counter-intuitive, because in food oils cold-pressing is a quality marker. For citrus peel oil on skin, it is the opposite.
The fragrance industry already treats this as a hard limit. The International Fragrance Association restricts furocoumarin-containing ingredients in leave-on skin products to very low total furocoumarin levels — on the order of a part per million — precisely because of phototoxicity. A commercial cosmetic containing citrus oil has been formulated to that constraint. Your kitchen-table blend has not.

Leaf Oil Versus Peel Oil

Makrut lime is unusual among citrus in being sold as two distinct essential oils, and the distinction matters for safety as much as for smell.

Leaf oil — steam-distilled from the double-lobed leaves. Chemistry is citronellal-led, with citronellol, linalool, sabinene, β-pinene, isopulegol, terpinen-4-ol and 1,8-cineole. A 2022 survey of citrus leaf essential oils by Clery and colleagues in Chemistry & Biodiversity identified citronellal as the characteristic compound of the Citrus hystrix group. Furocoumarins are principally a peel phenomenon, and steam distillation removes most of what little might be present, so distilled leaf oil is the lower-risk option for any leave-on preparation. It still carries ordinary irritation and sensitisation risk.
Peel oil — from the thick, bumpy rind. Limonene-led, with β-pinene, sabinene, γ-terpinene and a citronellal contribution. This is the oil that carries the phototoxicity question, and if it is expressed rather than distilled, it should be treated as phototoxic.

Bottom line for a home preparation: if you want a makrut-scented body or hair oil, use steam-distilled leaf oil, properly diluted. If you want peel oil for its brighter aroma, use it in something that is rinsed off, or in something you will not wear in daylight, or buy it steam-distilled or FCF.

One further note that affects the whole subject: makrut leaves lose most of their volatile oil on drying, and distillation itself selects for what carries over in steam. Dried leaf, fresh leaf, expressed peel oil and distilled leaf oil are four different materials. Safety information about one does not transfer automatically to another, and neither does aroma or chemistry.

Practical Rules for Skin and Hair

Concrete and specific, because vague advice is how people get hurt.

Keep expressed citrus peel oil off skin that will see sunlight for at least 12–18 hours. This is the core rule. Not “wear sunscreen” — sunscreen is imperfect UVA protection, is applied unevenly and wears off, and this reaction needs very little UVA.
Remember UVA passes through glass and cloud. An overcast day, a car window and an office window are not protection. A tanning bed is worse than sunlight.
Prefer rinse-off applications for peel oil. The traditional makrut hair wash is a rinse-off, which is one reason a centuries-old practice has not produced a catalogue of injuries. A leave-in oil is a different proposition entirely.
Never use citrus oil as a facial treatment before going outside. Face, neck, chest, ears and the backs of the hands are the highest-exposure sites and the least forgiving cosmetically, because that is where months-long pigmentation shows.
Do not apply it before a beach day, a hike, gardening or a run. This includes home-made insect repellent, which is an unfortunate collision: repellent is applied precisely when you are about to be outdoors. If you want a makrut repellent, use distilled leaf oil, not expressed peel oil — see the repellent page, and note that botanical repellents are short-acting anyway.
Wash your hands after zesting or juicing. Soap and water, thoroughly, including under the nails and around the cuticles. Most everyday cases start with unwashed hands.
Wear gloves for volume work. Processing a bag of fruit for zest or juice puts far more material on your skin than a recipe does.
Do not use citrus juice or oil as a hair lightener in the sun. It is an old practice, it works partly by exactly the photochemistry described above, and it exposes scalp, hairline, ears and neck.
Never on broken, inflamed or freshly shaved skin. Absorption is much higher and irritation much worse.
Never in or near eyes, and never on mucous membranes. If oil gets in an eye, flush with a bland carrier oil rather than water — essential oils are not water-soluble — then rinse with water and seek advice if pain or blurring persists.

Dilution and Patch Testing

Essential oils are concentrated to a degree most people underestimate. A single drop can represent a large quantity of plant material, and the difference between a pleasant blend and a chemical burn is arithmetic.

Dilution, in general terms

Never apply neat. Undiluted essential oil on skin causes irritation and substantially raises the risk of developing a lasting allergy to it. “Neat application” advice circulating online has no safety basis.
General adult body use: roughly 1–2% essential oil in a carrier — about 6 to 12 drops per 30 mL. Common carriers are jojoba, sweet almond, grapeseed, olive, sunflower and fractionated coconut oil.
Face and sensitive areas: lower — on the order of 0.5–1%, and citrus peel oil arguably not at all on the face if it is expressed.
Older adults, anyone with sensitive or eczema-prone skin, and anyone with a history of fragrance allergy: lower still, and patch test without exception.
Children: much lower, if at all. Concentrated monoterpene oils are not appropriate for infants or toddlers, and oils containing 1,8-cineole should never be applied near a small child's face — there are documented cases of respiratory distress from that exposure.
Blends do not dilute each other. Two oils at 2% each is a 4% total load, not 2%.
Water does not dilute oil. Adding oil to a bowl of water leaves undiluted droplets floating on the surface, which then contact skin at full strength. Dilution means a carrier oil, or a proper emulsifier.

Patch testing

Mix the blend at the concentration you actually intend to use.
Apply a small amount to the inner forearm or behind the ear.
Leave it for 24 hours — longer is better, since allergic contact reactions are delayed and can take 48 to 72 hours to appear.
Look for redness, itching, small bumps, burning or scaling. Any of these means stop.
A patch test does not test for phototoxicity. It is a test of irritation and allergy on covered skin. Phototoxicity requires ultraviolet light, and there is no safe way to test for it at home. Follow the peel-oil rules instead.
Repeat the patch test with each new bottle. Composition varies between batches and suppliers, and oxidised oil behaves differently from fresh.

Eating a plant does not predict tolerating its concentrated oil on skin. Makrut leaf in soup is a few micrograms of volatile oil, dispersed and swallowed. A 2% skin blend is orders of magnitude more, held against a barrier that absorbs it.

Contact Allergy and Oxidation

Phototoxicity is not the only skin risk, and it is worth keeping the two apart because they behave differently. Phototoxicity is a chemical reaction requiring light, can affect anyone on first exposure, and is dose-dependent. Allergic contact dermatitis is an immune response requiring prior sensitisation, recurs on every subsequent exposure, and can be triggered by tiny amounts once established.

Makrut lime oils contain several of the best-documented fragrance allergens in cosmetic dermatology:

Limonene — abundant in peel oil.
Linalool — present in leaf oil.
Citral, citronellal and citronellol — the citronellal family that defines makrut leaf oil.

Several of these are among the fragrance substances the European Union requires to be declared on cosmetic labels specifically because of contact-allergy risk. Their presence on that list is not decoration; it reflects patch-test clinic data.

The oxidation point is the practically useful one. A substantial body of Scandinavian dermatology research, published largely in Contact Dermatitis, has established that air-oxidised limonene and linalool are far more sensitising than the fresh compounds. Exposure to oxygen converts them into hydroperoxides, and it is the hydroperoxides that the immune system reacts to. Fresh limonene is a weak allergen; aged, air-exposed limonene is a significant one.

What that means for a bottle on your shelf:

Buy small bottles you will finish, rather than a large one you will keep for years.
Keep them tightly closed, cool and dark. Amber or cobalt glass, a fridge or a cool cupboard. Every opening admits oxygen.
Citrus oils have short shelf lives — commonly cited as around a year, less once opened frequently. They oxidise faster than most other essential-oil families.
Discard oil that smells sharp, turpentine-like, flat or “off,” or that has thickened or gone cloudy. That smell is oxidation, and it is telling you the allergen load has gone up.
Do not re-test your tolerance by using more. Once you are sensitised to a fragrance allergen, you are sensitised for life, and it will cross-react with a great many products you did not expect.

Never Internal

Stated without qualification, because this is where the most serious harm happens: makrut lime essential oil should never be taken internally. Not neat, not in water, not in a capsule, not a drop under the tongue, not in tea, not in honey, not in cooking.

The reasons:

No established oral dose exists for this species, from any authority. Nobody has determined a safe intake, because nobody has studied it.
Concentrated monoterpene oils injure mucosa. They can cause chemical burns of the mouth, pharynx, oesophagus and stomach.
Aspiration is a specific hazard. Volatile hydrocarbon oils that reach the airway can cause a chemical pneumonitis, which is why ingestion of monoterpene-rich oils is a recognised poisoning presentation and why inducing vomiting after such an ingestion is the wrong move.
Children are at serious risk. Small volumes of concentrated essential oil have caused significant toxicity in children. Keep bottles capped, out of reach and out of sight, and prefer bottles with child-resistant closures and integral drop restrictors.
“Food grade” and “therapeutic grade” are marketing terms, not regulatory categories, and neither implies that swallowing a concentrated oil is safe.

If you swallow essential oil, or a child does, contact a poisons information centre or emergency service immediately. Do not induce vomiting. Take the bottle with you.

None of this applies to culinary use of the plant. Makrut leaves and zest in cooking are food, they are used across Southeast Asia daily, and they are unremarkable. The distinction is between a leaf and a distillate.

Pregnancy, Breastfeeding and Children

The right distinction here is not “makrut lime: safe or unsafe?” but which form.

Culinary leaf and zest in pregnancy and breastfeeding: customary and unremarkable. Torn leaves in soup, shreds over a salad, zest in a curry paste — this is ordinary food throughout a region where a great many people are pregnant at any given moment, and there is no signal of harm. Nothing on this page is a reason to avoid Thai food.
Concentrated essential oil in pregnancy: not studied, so avoid internally and be conservative topically. No adequate safety data exists for Citrus hystrix oil in pregnancy. The reasonable position is: never internally; topically only well diluted, and skipping it is a perfectly good option; and remember that furocoumarin phototoxicity plus the increased tendency to pigmentation in pregnancy is a poor combination on the face.
Breastfeeding. Culinary use is fine. Keep concentrated oil off the chest and nipple area, where an infant will make direct oral contact.
Infants and young children. Food use is fine. Concentrated essential oils should be kept away from infants and young children entirely, and never applied near the face — oils containing 1,8-cineole in particular have caused respiratory distress in small children. Store bottles where children cannot reach them.
A note on inhalation. Diffused oils in a room with a baby, a person with asthma, or a bird are worth avoiding. Aerosolised terpenes can irritate airways, and birds have exceptionally sensitive respiratory systems.

Furocoumarins and Medication

The same compound family that causes phototoxicity on skin has a second, unrelated relevance inside the body: citrus furocoumarins inhibit intestinal CYP3A4, the enzyme responsible for first-pass metabolism of a great many drugs. This is the grapefruit-juice interaction, and it can raise blood levels of certain statins, some calcium-channel blockers, certain immunosuppressants and a long list of others, sometimes substantially.

Applied honestly to makrut lime:

Makrut peel has not been specifically quantified for the furocoumarins that drive this interaction, and no case of a makrut-related drug interaction appears to have been reported.
Culinary amounts are very small. The zest in a curry paste serving four to six people is a fraction of a fruit's rind. This is not the same exposure as a daily glass of grapefruit juice.
Concentrated peel preparations are a different matter. An extract, a tincture of peel, or a supplement is not a curry.
If you take a drug carrying a strong grapefruit warning, that is a reason to be moderate with concentrated peel preparations and to ask your pharmacist — not a reason to give up Thai food. Pharmacists answer this question well and for free.

Tier: the CYP3A4 mechanism is established clinical pharmacology; its application to makrut lime specifically is inference from the plant family, and is stated as such.

Buying, Labelling and Storing

A short checklist for anyone buying makrut lime essential oil.

Which part? The label should say leaf or peel (or rind / zest). If it does not, that is a poor sign. They are different products with different chemistry and different risk.
Which method? Steam-distilled or cold-pressed/expressed. For anything leave-on, prefer steam-distilled.
Botanical name. Citrus hystrix should appear. Mislabelling between citrus species is common in the aromatics trade.
Country of origin and batch or lot number. Both are marks of a supplier who can answer questions about composition.
Packaging. Amber or cobalt glass, tight cap, ideally child-resistant with a drop restrictor. Never buy essential oil in clear glass or plastic.
Small volume. Buy what you will use within a year. Citrus oils oxidise quickly.
Ignore “therapeutic grade” and “food grade.” Neither is a regulatory standard; both are marketing.
Storage. Cool, dark, sealed. A refrigerator is genuinely better than a shelf for citrus oils. Write the opening date on the bottle.

If a Reaction Happens

General first-aid information, not a substitute for medical advice.

Get out of the sun and stay out of it while the reaction runs its course. Cover the area with clothing rather than relying on sunscreen.
Wash the area with soap and water to remove any remaining oil, then leave it alone.
Cool compresses and bland emollients are the usual comfort measures. Do not apply more essential oil to it, and do not apply anything “natural” and untested to damaged skin.
Do not deliberately break blisters. Intact blister roof is a sterile dressing.
See a clinician if blistering is extensive; the face, eyes or genitals are involved; there is significant pain; there are signs of infection (increasing redness, warmth, pus, fever); or you are unsure what caused it. A short course of a topical corticosteroid is often prescribed for the inflammatory phase, and that is a decision for a clinician.
Expect the pigmentation to linger. Post-inflammatory hyperpigmentation from phytophotodermatitis commonly persists for weeks to many months. Strict sun protection of the affected area is the single most useful thing for helping it fade and stopping it darkening further.
Tell the story clearly. The bizarre streaky pattern is frequently misread. Saying “I had lime oil on my hands and then spent the afternoon outside” is diagnostic information that saves a lot of guessing.

Key Research Papers

Every citation links to a PubMed topic search rather than a numeric record, so a mistyped identifier can never point you at the wrong paper. Titles, journals and years appear in the text where a specific paper is named; elsewhere a body of literature is described and the search runs against it.

Phytophotodermatitis from limes and citrus peel — the clinical literature of case reports and reviews describing the delayed erythema, blistering and long-lasting hyperpigmentation, including the “margarita burn” pattern. Tier: established clinical dermatology. Run the topic search
Furocoumarins, bergapten and the phototoxicity of expressed citrus peel oils — the chemistry and testing behind the rule, and the basis for restricting these compounds in leave-on cosmetics. Tier: mechanistic and regulatory science. Run the topic search
Psoralen photochemistry and DNA cross-linking — how a psoralen plus UVA damages a cell, established through decades of PUVA research. Tier: mechanistic. Run the topic search
PUVA therapy — psoralen plus UVA in dermatology — the deliberate clinical use of this reaction, which is the strongest possible evidence that the mechanism is real. Tier: randomized clinical trials and long-term clinical practice. Run the topic search
Bullous phototoxic reactions to bergamot and citrus aromatherapy oils — case reports in the dermatology literature of blistering after topical citrus oil and sun exposure; the closest published analogue to a home-made makrut body oil. Tier: clinical case reports. Run the topic search
Oxidised limonene and linalool as contact allergens — the body of Scandinavian dermatology research, largely in Contact Dermatitis, showing that air-aged terpenes form sensitising hydroperoxides. Tier: clinical dermatology and patch-test research. Run the topic search
Fragrance contact allergy and the EU labelled allergens — why citral, citronellol, limonene and linalool are declared on cosmetic labels. Tier: clinical epidemiology. Run the topic search
Chemical diversity of citrus leaf essential oils — Clery and colleagues, Chemistry & Biodiversity, 2022. Establishes citronellal as the characteristic Citrus hystrix leaf-oil compound, and the reason leaf oil and peel oil must be considered separately. Tier: analytical chemistry. Find on PubMed
Chemical composition and antimicrobial activity of the essential oils from New Caledonian Citrus macroptera and Citrus hystrix — Waikedre and colleagues, Chemistry & Biodiversity, 2010. Demonstrates how far makrut leaf-oil composition can vary between chemotypes. Tier: analytical chemistry. Find on PubMed
Citrus peel furocoumarins and CYP3A4 inhibition — the grapefruit-interaction mechanism, relevant to concentrated peel preparations. Tier: clinical pharmacology. Run the topic search
Essential-oil poisoning and paediatric ingestion — the toxicology literature behind “never internally” and behind keeping bottles out of children's reach. Tier: clinical toxicology. Run the topic search
1,8-Cineole and eucalyptus-type oils applied near the face of young children — the respiratory-distress reports behind that specific prohibition. Tier: clinical case reports. Run the topic search
Post-inflammatory hyperpigmentation — why the mark left by a phototoxic reaction lasts so long and why sun protection is the main intervention. Tier: clinical dermatology. Run the topic search
Giant hogweed and Apiaceae phytophotodermatitis — the same chemistry in another plant family, and the source of the most severe reported cases. Useful for calibrating how serious psoralens can be. Tier: clinical case reports and reviews. Run the topic search

Connections

All Herbs
Makrut Lime (Citrus hystrix) — the main topic page: botany, names, compounds, forms and cautions.
Makrut Lime Benefits Hub — the four deep dives and how the evidence tiers work.
Aroma, Stress and Insect Repellent Use — where the “apply before going outside” conflict arises.
Antimicrobial and Oral-Health Research — the scalp and oral applications people most often improvise with oil.
Digestion and Bloating — and why the answer there is a leaf, never the oil.
Contact Dermatitis — the allergic reaction that is distinct from phototoxicity.
Eczema — a damaged barrier absorbs far more of anything applied to it.
Psoriasis — the condition PUVA therapy was developed for, using this very photochemistry deliberately.
Seborrheic Dermatitis — the scalp condition behind the traditional makrut hair wash.
Actinic Keratosis — the longer-term consequence of cumulative ultraviolet damage.
Dermatology — the full skin-conditions index.
Tea Tree — the essential oil with the best-documented contact-allergy literature, oxidation included.
Lemongrass — citral-rich, and subject to the same fragrance-allergen cautions.
Eucalyptus — the 1,8-cineole oil behind the paediatric face-application warning.
Lavender — linalool-rich, and another oil whose oxidised form is the sensitiser.
Chen Pi — citrus peel used as a medicine, and the same furocoumarin family.
Clove — eugenol, another concentrated oil routinely misused neat.
Aromatherapy — where these preparations are used, and the safety practices that go with them.
Massage — carrier oils, dilution and the setting where phototoxic reactions have been reported.
Hesperidin — the non-volatile citrus flavonoid fraction, distinct from the essential oil.

Safety and disclaimer. This page is general information, not medical advice, and it is not a formulation manual. Cold-pressed citrus peel oil on skin followed by ultraviolet exposure can cause a genuine chemical burn with pigmentation lasting months; the reaction is delayed by 12 to 24 hours, so you cannot judge it as it happens. Essential oils must never be swallowed and never applied undiluted, and must be kept away from infants and young children. Culinary use of makrut lime leaf and zest is ordinary food and is not what this page is about. If you develop a blistering, streaky or unexplained skin reaction — especially on the face, eyes or genitals, or with signs of infection — see a clinician. If essential oil is swallowed, contact a poisons information centre or emergency service immediately and do not induce vomiting. If you are pregnant, breastfeeding, treating a child, or taking prescription medication, speak to a doctor or pharmacist before using concentrated plant preparations.

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