Nicotine Metabolites and Chemistry
The liver turns most nicotine into cotinine through the enzyme CYP2A6, and how fast each person does it shapes how much they use. Some metabolites, such as cotinine and 6-hydroxy-nicotine, are studied as active compounds in their own right. This page collects the published research on nicotine's metabolism, metabolites and chemistry, each paper summarised in plain language with its PubMed record.
This collection is research only: papers found on PubMed, each described as its own abstract reports it. Cell and animal results are labelled as such, and harms are listed beside benefits. It is not medical advice.
Table of Contents
Research at a Glance
What kind of study each paper on this page is, and when the papers were published. Hover over a bar for its count.
The Papers (22)
Newest first. Study types on this page — Human trial: 3 · Human observational study: 3 · Review: 4 · Animal study: 7 · Chemistry / laboratory method: 4 · Other: 1. Each summary is written from the paper’s own abstract; follow the PubMed link for the full record.
Nicotine metabolites generate craniofacial defects that are worsened by ethanol exposure
Animal study, 2026. Researchers studied zebrafish embryos, which break down nicotine in a way similar to humans, to learn how nicotine exposure affects the development of the face and skull cartilage. They found that the damage came from nicotine's breakdown products (metabolites), mainly one called 4HPBA and then nicotine-N'-oxide, and not from nicotine itself. Zebrafish lacking the gene for the enzyme that breaks down nicotine (the fish version of the human enzyme CYP2A6) had lower metabolite levels and were partly protected from these defects. When embryos were exposed to alcohol and nicotine together, they made fewer metabolites and were less affected than with nicotine alone, but alcohol made the defects worse in embryos exposed directly to the metabolites; these results come from zebrafish only.
Borrego-Soto G, Riddington IM, Maier E et al. (2026). Nicotine metabolites generate craniofacial defects that are worsened by ethanol exposure. PNAS Nexus. — PubMed PMID: 42808074 · doi:10.1093/pnasnexus/pgag300
The Gas Phase Protonation Sites of Six Naturally Occurring Nicotinoids
Chemistry / laboratory method, 2024. Researchers used specialized cold-ion trap infrared spectroscopy and computer modeling to study six naturally occurring nicotine-related molecules, including nicotine itself, nornicotine, anabasine, anatabine, cotinine, and myosmine. They determined exactly where on each molecule a hydrogen ion (proton) attaches, finding that nicotine exists as a mix of two different attachment forms, while most of the others attach in just one spot and myosmine attaches very differently due to its distinct molecular shape. This is a purely chemical/molecular-structure study, not a study in cells, animals, or people.
Okura Y, Santis GD, Hirata K et al. (2024). The Gas Phase Protonation Sites of Six Naturally Occurring Nicotinoids. J Phys Chem Lett. — PubMed PMID: 38940770 · doi:10.1021/acs.jpclett.4c01206
Predicting nicotine metabolism across ancestries using genotypes
Human observational study, 2022. Researchers used genetic and metabolite data from four studies covering people of multiple ancestries (one group of 2,239 people, validated against three more groups totaling 1,415 people) to build computer models predicting how fast someone's body breaks down nicotine, based on DNA markers plus age, sex, height and weight. The models' predictions matched actual measured nicotine breakdown fairly well, with the combined model performing differently across ancestry groups (African, Asian, European), and the strongest genetic predictors were variants near the CYP2A6 gene along with age and ancestry.
Baurley JW, Bergen AW, Ervin CM et al. (2022). Predicting nicotine metabolism across ancestries using genotypes. BMC Genomics. — PubMed PMID: 36131240 · doi:10.1186/s12864-022-08884-z
Subchronic effects of plant alkaloids on anxiety-like behavior in zebrafish
Animal study, 2021. Researchers tested whether nicotine and several related plant alkaloids could reduce anxiety-like behavior in zebrafish, using a test that measures how much the fish explore a new tank versus staying at the bottom. Nicotine and the comparison drug buspirone both reduced this anxiety-like behavior after a single dose and still worked after 5 days of repeated exposure. Cotinine, the main breakdown product of nicotine, had a similar calming effect but needed a higher dose, while two other nicotine-related alkaloids (anabasine and anatabine) and two other natural compounds (harmane and norharmane) did not show this effect. The researchers concluded that nicotine had a distinctive, consistent anxiety-reducing effect in zebrafish that was not shared by the closely related alkaloids tested.
Hawkey AB, Hoeng J, Peitsch MC et al. (2021). Subchronic effects of plant alkaloids on anxiety-like behavior in zebrafish. Pharmacol Biochem Behav. — PubMed PMID: 34197843 · doi:10.1016/j.pbb.2021.173223
Self-Administration of Cotinine in Wistar Rats: Comparisons to Nicotine
Animal study, 2021. Rats were trained to self-administer either cotinine (a byproduct the body makes from nicotine) or nicotine itself through an IV line. The rats did self-administer cotinine, working harder for it as the task got harder, with blood levels ranging from 77 to 792 ng/ml, and there was no difference between male and female rats. Nicotine still produced stronger effects than cotinine at comparable doses, and two drugs that blocked nicotine's effects on nicotinic receptors did not block cotinine's effects, suggesting cotinine may work partly through a different route.
Ding ZM, Gao Y, Sentir AM et al. (2021). Self-Administration of Cotinine in Wistar Rats: Comparisons to Nicotine. J Pharmacol Exp Ther. — PubMed PMID: 33361363 · doi:10.1124/jpet.120.000367
Nicotine absorption from e-cigarettes over 12 months
Human observational study, 2019. Researchers followed 27 experienced e-cigarette users, measuring their saliva for a nicotine breakdown product (cotinine) at the start and again 12 months later. Over that year, people lowered the nicotine strength in their e-liquid (from about 13.83 mg/mL to 9.91 mg/mL) but drank more of it each day (from 4.44 mL to 6.84 mL), and their saliva cotinine levels stayed about the same (370.88 vs 415.78 ng/mL), suggesting they kept their overall nicotine intake steady by vaping more intensely even as the liquid got weaker.
Soar K, Kimber C, McRobbie H et al. (2019). Nicotine absorption from e-cigarettes over 12 months. Addict Behav. — PubMed PMID: 30054021 · doi:10.1016/j.addbeh.2018.07.019
Nicotine enhances alcoholic fatty liver in mice: Role of CYP2A5
Animal study, 2018. This study used CYP2A5 knockout mice and normal littermates to see whether a liver enzyme that breaks down nicotine affects how nicotine worsens alcohol-related fatty liver. In mice with the enzyme, nicotine and its metabolite cotinine made alcoholic fatty liver worse and raised blood triglycerides and markers of oxidative stress; in mice lacking the enzyme, nicotine and cotinine had no such worsening effect. The researchers also found that this enzyme produced reactive oxygen species while breaking down nicotine and cotinine, suggesting this metabolic process is what drives the liver damage.
Chen X, Owoseni E, Salamat J et al. (2018). Nicotine enhances alcoholic fatty liver in mice: Role of CYP2A5. Arch Biochem Biophys. — PubMed PMID: 30222954 · doi:10.1016/j.abb.2018.09.012
Discriminative Stimulus Properties of S(-)-Nicotine: "A Drug for All Seasons"
Review, 2018. This review discusses a laboratory method used in animals and humans in which subjects learn to tell apart the internal state produced by nicotine from a no-drug state. It describes how nicotine's effects in this test depend on its exact molecular shape (one mirror-image form is more potent than the other), how the effects are blocked by drugs that act on a specific type of nicotine receptor in the brain, and how dopamine brain chemicals are likely also involved. It notes that two anti-smoking medications may work partly because they produce nicotine-like effects in this same test, and it proposes using this method to study staying off nicotine products long-term.
Rosecrans JA, Young R (2018). Discriminative Stimulus Properties of S(-)-Nicotine: "A Drug for All Seasons". Curr Top Behav Neurosci. — PubMed PMID: 28391535 · doi:10.1007/7854_2017_3
Detoxification and elimination of nicotine by nectar-feeding birds
Animal study, 2017. Researchers studied two nectar-feeding bird species, white-bellied sunbirds and Cape white-eyes, to see how they handle nicotine found naturally in some flower nectar. A high nicotine concentration reduced food intake and caused dehydration in both species, but the birds broke down nicotine efficiently and excreted very little of it unchanged. The two species used different chemical pathways to process nicotine: white-eyes mainly produced norcotinine while sunbirds mainly produced nornicotine, and white-eyes relied more on a particular detox pathway (mercapturic acid conjugation) than sunbirds did.
Lerch-Henning S, Du Rand EE, Nicolson SW (2017). Detoxification and elimination of nicotine by nectar-feeding birds. J Comp Physiol B. — PubMed PMID: 28150179 · doi:10.1007/s00360-016-1055-4
Electronic cigarettes: The nicotyrine hypothesis
Other, 2015. This paper puts forward a hypothesis and does not report new experiments. It proposes that nicotyrine, a compound that forms slowly when nicotine in e-cigarette liquid is exposed to air, is inhaled along with nicotine and blocks the CYP2A enzymes in the airways and liver that break nicotine down into cotinine. The authors suggest this could slow nicotine clearance and ease withdrawal symptoms, so smaller, less frequent doses could keep nicotine levels up in people who use an e-cigarette. They note possible risks, such as slower clearance of other substances handled by these enzymes, and suggest that blocking these enzymes might one day help people stop smoking.
Abramovitz A, McQueen A, Martinez RE et al. (2015). Electronic cigarettes: The nicotyrine hypothesis. Med Hypotheses. — PubMed PMID: 26100465 · doi:10.1016/j.mehy.2015.06.002
Nicotine chemistry, metabolism, kinetics and biomarkers
Review, 2009. This review looks at how nicotine is absorbed, spread through the body and cleared, both from tobacco and from medicinal nicotine products used to help people quit smoking. Nicotine is broken down mainly in the liver by the enzymes CYP2A6, UGT and FMO, and how fast this happens depends on genes, diet and meals, age, sex, estrogen-containing hormone products, pregnancy, kidney disease, other medicines and smoking itself, with large differences between racial and ethnic groups. Cotinine, a breakdown product of nicotine, is the most widely used marker of nicotine intake and can be measured in blood, urine, saliva, hair or nails. The authors report that the best blood cotinine cut-off to tell smokers from non-smokers in the general US population is now 3 ng/ml, much lower than the cut-off set 20 years earlier, which they link to less secondhand smoke exposure and more light or occasional smoking.
Benowitz NL, Hukkanen J, Jacob P (2009). Nicotine chemistry, metabolism, kinetics and biomarkers. Handb Exp Pharmacol. — PubMed PMID: 19184645 · doi:10.1007/978-3-540-69248-5_2
Nicotine dependence pharmacogenetics: role of genetic variation in nicotine-metabolizing enzymes
Review, 2009. This review looks at how inherited differences in the body's nicotine-processing enzymes affect smoking behavior and how well quit-smoking treatments work. The liver enzyme CYP2A6 does most of the work of breaking down nicotine, and people who carry gene versions that make this enzyme weak or inactive tend to smoke fewer cigarettes per day, are less dependent on nicotine, and may find quitting easier. A blood measure of how fast someone breaks down nicotine, the ratio of two nicotine breakdown products (3'-hydroxycotinine to cotinine), predicted success with the nicotine patch and with counseling alone; faster metabolizers did worse with these and may be better suited to non-nicotine medicines such as bupropion. The authors also describe slowing CYP2A6 to keep nicotine in the body longer as a promising way to reduce harm from smoking.
Ray R, Tyndale RF, Lerman C (2009). Nicotine dependence pharmacogenetics: role of genetic variation in nicotine-metabolizing enzymes. J Neurogenet. — PubMed PMID: 19169923 · doi:10.1080/01677060802572887
Monoamine oxidase A rather than monoamine oxidase B inhibition increases nicotine reinforcement in rats
Animal study, 2006. Rats were trained to self-administer nicotine and then treated with different drugs that block an enzyme called monoamine oxidase (MAO), which tobacco smoke is known to inhibit. A drug that blocked MAO-A increased how much nicotine the rats took in, and under a more demanding testing schedule, both the MAO-A blocker and one MAO-B blocker (norharmane) made nicotine more rewarding, while a different MAO-B blocker (selegiline) had no effect. The researchers concluded that blocking MAO-A enhances nicotine's rewarding effects in rats, suggesting other tobacco smoke chemicals besides nicotine may contribute to smoking behavior.
Guillem K, Vouillac C, Azar MR et al. (2006). Monoamine oxidase A rather than monoamine oxidase B inhibition increases nicotine reinforcement in rats. Eur J Neurosci. — PubMed PMID: 17229101 · doi:10.1111/j.1460-9568.2006.05217.x
Metabolism and disposition kinetics of nicotine
Review, 2005. This review covers what is known about how the body absorbs, distributes, breaks down and gets rid of nicotine, along with some related tobacco alkaloids and nicotine-like compounds being developed as medicines. It mainly covers studies in people, and brings in animal data where it helps explain the human findings. It describes the enzymes that break down nicotine, with a special focus on the liver enzyme CYP2A6, which does most of the work of clearing nicotine. It also looks at what changes how fast nicotine is broken down, including genetic differences, diet, age, sex, pregnancy, liver and kidney disease, racial and ethnic background, smoking, and medicines such as oral contraceptives.
Hukkanen J, Jacob P, Benowitz NL (2005). Metabolism and disposition kinetics of nicotine. Pharmacol Rev. — PubMed PMID: 15734728 · doi:10.1124/pr.57.1.3
Screening for drugs of abuse in hair with ion spray LC-MS-MS
Human observational study, 2004. Researchers tested a new lab method (LC-MS-MS) that can screen hair samples for many drugs at once, including nicotine and its breakdown product cotinine, alongside opioids, stimulants and other drugs. Using hair from 75 autopsy cases, the method found 26 positive drug results in 16 of the cases, and the authors concluded this screening approach was fast, sensitive and well suited to postmortem drug testing.
Kronstrand R, Nyström I, Strandberg J et al. (2004). Screening for drugs of abuse in hair with ion spray LC-MS-MS. Forensic Sci Int. — PubMed PMID: 15451091 · doi:10.1016/j.forsciint.2004.04.034
Nicotine-related alkaloids and metabolites as inhibitors of human cytochrome P-450 2A6
Chemistry / laboratory method, 2004. This laboratory study tested nicotine and 13 related tobacco alkaloids and nicotine breakdown products for how strongly they block CYP2A6, the main human liver enzyme that breaks down nicotine, using the human enzyme made in the lab rather than people. Nicotine and anatabine were fairly strong blockers (Ki values of 4.4 and 3.8 microM), while most of the others, including cotinine and nornicotine, were much weaker (20 to more than 300 microM). Beta-nicotyrine, a nicotine-related compound present after smoking, was by far the strongest blocker (Ki 0.37 microM), and only nicotine and beta-nicotyrine permanently switched off the enzyme as it worked on them. The authors suggest that in smokers these compounds may slow this enzyme more than their blood levels alone would predict.
Denton TT, Zhang X, Cashman JR (2004). Nicotine-related alkaloids and metabolites as inhibitors of human cytochrome P-450 2A6. Biochem Pharmacol. — PubMed PMID: 14757175 · doi:10.1016/j.bcp.2003.10.022
Nicotine increases hepatic oxygen uptake in the isolated perfused rat liver by inhibiting glycolysis
Animal study, 2002. Researchers infused nicotine into isolated, perfused rat livers and into liver cells taken from rats. Nicotine increased oxygen uptake by the liver (10% at a lower dose, 27% at a higher dose) while slowing glycolysis, but had no effect in livers depleted of glycogen by fasting. The effect depended on nicotinic acetylcholine receptors, calcium changes inside liver cells, and a cell-signaling pathway (cyclic AMP/protein kinase A), supporting the idea that nicotine raises liver oxygen use by slowing glycolysis and boosting mitochondrial respiration.
Dewar BJ, Bradford BU, Thurman RG (2002). Nicotine increases hepatic oxygen uptake in the isolated perfused rat liver by inhibiting glycolysis. J Pharmacol Exp Ther. — PubMed PMID: 12023521 · doi:10.1124/jpet.301.3.930
The influence of a transmucosal cholinergic agonist on pharyngeal muscle activity
Human trial, 2000. Researchers gave normal-weight men a nicotine patch attached to a tooth to study its effect on throat muscles while awake, testing two groups of 12 and 13 people. Nicotine blood levels rose significantly after the patch, but none of the measured throat-muscle activity or airway responses changed significantly overall, though a few participants showed temporary increases that faded as the nicotine wore off. The researchers concluded that oral nicotine failed to consistently activate the tongue muscle, possibly because not enough nicotine reached the muscle itself.
Slamowitz DI, Edwards JK, Chajek-Shaul T et al. (2000). The influence of a transmucosal cholinergic agonist on pharyngeal muscle activity. Sleep. — PubMed PMID: 10875561 · doi:10.1093/sleep/23.4.543
Optimization study for the reversed-phase ion-pair liquid chromatographic determination of nicotine in commercial tobacco products
Chemistry / laboratory method, 1999. This laboratory study looked at how to measure nicotine accurately in commercial tobacco products using a technique called liquid chromatography, which separates the chemicals in a sample. The researchers studied how nicotine and several related tobacco alkaloids (anatabine, nornicotine, anabasine and cotinine) behave in this test, noting that nicotine is a base with two acid-base values of 3.12 and 8.02. They used what they learned about each chemical's size, structure and acid-base strength to explain the order in which the chemicals come out of the test, and to choose the best test conditions for measuring nicotine. No people or animals were involved; this was a chemistry method study.
Ciolino LA, Turner JA, McCauley HA et al. (1999). Optimization study for the reversed-phase ion-pair liquid chromatographic determination of nicotine in commercial tobacco products. J Chromatogr A. — PubMed PMID: 10481983 · doi:10.1016/s0021-9673(99)00639-1
Transdermal nicotine inhibits interleukin 2 synthesis by mononuclear cells derived from healthy volunteers
Human trial, 1998. Twelve healthy non-smoking volunteers wore nicotine patches for two weeks (building up to 15 mg per day) while another twelve wore placebo patches, and researchers tested blood cells taken before and during treatment. After two weeks, the nicotine group's immune cells produced significantly less interleukin-2 than the placebo group, and production of two other immune signals, interleukin-10 and TNF-alpha, also dropped compared to the start of treatment.
van Dijk AP, Meijssen MA, Brouwer AJ et al. (1998). Transdermal nicotine inhibits interleukin 2 synthesis by mononuclear cells derived from healthy volunteers. Eur J Clin Invest. — PubMed PMID: 9767362 · doi:10.1046/j.1365-2362.1998.00344.x
Cotinine determination by immunoassays may be influenced by other nicotine metabolites
Chemistry / laboratory method, 1988. Researchers tested a laboratory antibody test used to measure cotinine, a breakdown product of nicotine, in body fluids as a way to track nicotine exposure. They found the antibody also reacted with another nicotine breakdown product, trans-3'-hydroxycotinine, by about 30%, and since smokers excrete three times more of that substance than cotinine in urine, this cross-reaction could make urine cotinine test results read higher than the true amount.
Schepers G, Walk RA (1988). Cotinine determination by immunoassays may be influenced by other nicotine metabolites. Arch Toxicol. — PubMed PMID: 3242451 · doi:10.1007/BF00293630
Trans-3'-hydroxycotinine as a main metabolite in urine of smokers
Human trial, 1987. Nine male smokers smoked three cigarette types with different nicotine levels over several days, with urine collected throughout. Nicotine and four known breakdown products (cotinine, nicotine-N'-oxide, nornicotine, N-methyl-nicotinium) were found, but trans-3'-hydroxycotinine was present in even higher amounts in urine than nicotine or cotinine, and it was the second most abundant nicotine breakdown product in blood after cotinine.
Neurath GB, Dünger M, Orth D et al. (1987). Trans-3'-hydroxycotinine as a main metabolite in urine of smokers. Int Arch Occup Environ Health. — PubMed PMID: 3557628 · doi:10.1007/BF00378497
PubMed Topic Searches
More Nicotine Research
All nicotine research →Every page in the collection, with how many papers each one lists.
Connections
- How Your Liver Actually Detoxifies — interactive animation
- Nicotine: The Research
- The Nicotine Patch, animated
- Nicotinic Acetylcholine Receptors
- Cigarette Smoke — what burnt tobacco adds that nicotine alone does not
- Brain and Memory
- Mood and Mental Health
- Inflammation and Immunity
- Cancer Research
- Heart and Blood Vessels
- Metabolism and Weight
- Addiction and Withdrawal
- Patches, Gum and Delivery