Benzathine Penicillin G: Syphilis Treatment by Stage

Table of Contents

1. Why T. pallidum Has Never Developed Penicillin Resistance
2. Primary, Secondary, and Early Latent: 2.4M Units IM ×1
3. Late Latent and Unknown Duration: Weekly ×3
4. Tertiary and Neurosyphilis: Aqueous Penicillin G Intravenous
5. The Jarisch-Herxheimer Reaction: Mechanism and Management
6. Alternatives for Penicillin Allergy
7. Treatment Failure and Retreatment
8. Monitoring Treatment: RPR Titer Decline
9. Key Research Papers
10. Featured Videos

1. Why T. pallidum Has Never Developed Penicillin Resistance

In 80-plus years of widespread penicillin use, Treponema pallidum has never acquired clinically significant resistance to penicillin. This is exceptional. Neisseria gonorrhoeae, another sexually transmitted bacterium, developed penicillin resistance within years of penicillin's introduction and is now resistant to multiple antibiotic classes. T. pallidum has not budged.

The reasons are not fully understood but probably involve several factors:

• Minimal genome, minimal adaptability. T. pallidum has one of the smallest genomes of any pathogenic bacterium (~1.14 Mb, encoding ~1,000 genes). It has discarded most of its metabolic capabilities, outsourcing them to the host. This stripped-down genome leaves little room for acquiring or maintaining resistance genes, which typically require additional metabolic machinery.
• Long impossible to grow in the lab. For more than a century T. pallidum could not be grown continuously outside a living host, which held back laboratory study of the organism. In 2018 researchers reported long-term growth in the laboratory alongside rabbit epithelial cells, with subculture every 6 to 7 days and continuous growth for over 6 months while the bacteria stayed infectious (Edmondson 2018). The authors expect this culture system to make it possible to study the organism's antibiotic susceptibility directly.
• No horizontal gene transfer detected. Many bacteria spread resistance genes through plasmids and other mobile genetic elements shared between organisms. This mechanism has not been documented for T. pallidum.
• Penicillin mechanism is difficult to circumvent. Penicillin kills bacteria by binding to penicillin-binding proteins (PBPs) and blocking cell wall synthesis. Resistance requires either producing beta-lactamase enzymes (which destroy penicillin) or altering PBPs so penicillin cannot bind. T. pallidum appears unable to accomplish either.

This sustained susceptibility is one of the most important facts in infectious disease. Unlike virtually every other major bacterial pathogen, syphilis treatment has not had to chase resistance.

2. Primary, Secondary, and Early Latent Syphilis: 2.4M Units IM ×1

For all early-stage syphilis (primary chancre, secondary rash, or early latent infection within one year of acquisition), the treatment is a single intramuscular injection of benzathine penicillin G 2.4 million units.

Practical details:

• Formulation: Benzathine penicillin G alone — not procaine penicillin G and not amoxicillin, which achieve different pharmacokinetic profiles. Dispensing errors (using the combined benzathine-procaine penicillin product, which contains procaine penicillin and provides half the benzathine dose) have caused treatment failures and are a recognized safety issue.
• Injection site: Intramuscularly into the gluteal muscle or the vastus lateralis (outer thigh). The injection is viscous and may cause brief discomfort.
• Pharmacokinetics: Benzathine penicillin provides sustained low-level penicillin G concentrations for two to three weeks from a single injection. This is precisely what is needed: T. pallidum divides slowly (approximately every 30 to 33 hours), and penicillin must be present continuously long enough to kill all organisms.
• Efficacy: A single dose cures primary and secondary syphilis in approximately 95% or more of cases. Early latent syphilis cure rates are somewhat lower and require careful serologic follow-up.
• One dose or three: A 2025 randomized trial of 249 people with early syphilis (61% living with HIV) compared one injection of 2.4 million units with three weekly injections. A serologic response at 6 months (the blood test turning negative or the RPR titer falling by two or more dilutions) was seen in 76% after one dose and 70% after three doses, so one dose was judged noninferior. The response was 76% with and 76% without HIV in the one-dose group, and no clinical relapse or treatment failure occurred in either group (Hook 2025).

3. Late Latent and Unknown Duration: Weekly ×3

For late latent syphilis (more than one year after infection) or syphilis of unknown duration, the standard regimen is three weekly injections of benzathine penicillin G 2.4 million units IM, one week apart (weeks 0, 1, and 2), for a total of 7.2 million units.

Why three doses instead of one?

• The drug concentration from a single injection may wane before it has had sufficient time to eliminate spirochetes that are sequestered in less vascular tissues (bone, fibrous scar tissue, the central nervous system).
• With longer duration of infection before treatment, organisms are more widely distributed and potentially in more metabolically quiescent states.
• The three-dose regimen provides 21 continuous days of treponemicidal drug levels from the first injection, with the second and third doses extending coverage.

On late doses, the CDC 2021 guideline states that, from clinical experience, a gap of 10–14 days between doses might be acceptable before the sequence of injections is restarted, and that 7–9 days might be preferred on pharmacologic grounds. In pregnancy, a gap of more than 9 days between doses means the full course is repeated.

4. Tertiary and Neurosyphilis: Aqueous Penicillin G Intravenous

Neurosyphilis and other forms of tertiary syphilis require a fundamentally different treatment approach: intravenous aqueous crystalline penicillin G at high doses for 10 to 14 days.

Why not benzathine penicillin G for neurosyphilis? Benzathine penicillin G achieves adequate blood and tissue levels but does not reliably penetrate the blood-brain barrier at treponemicidal concentrations. Studies measuring penicillin levels in cerebrospinal fluid after benzathine penicillin G injection found concentrations often below the minimum inhibitory concentration for T. pallidum. The intravenous formulation, by contrast, achieves cerebrospinal fluid (CSF) levels well above the minimum inhibitory concentration (MIC).

The neurosyphilis regimen:

• Aqueous crystalline penicillin G 18–24 million units per day, given either as 3 to 4 million units intravenously every 4 hours or as a continuous intravenous infusion, for 10 to 14 days
• Alternative if intravenous access is difficult: Procaine penicillin G 2.4 million units IM daily PLUS probenecid 500 mg orally four times daily, both for 10 to 14 days. Probenecid blocks renal tubular secretion of penicillin, increasing and prolonging blood levels to achieve adequate central nervous system (CNS) penetration.
• After completing intravenous treatment: because the neurosyphilis regimens are shorter than the latent-syphilis regimen, the CDC 2021 guideline states that benzathine penicillin G 2.4 million units IM once a week for 1–3 weeks “can be considered” afterwards to provide a comparable total duration of therapy.

For ocular syphilis (uveitis, retinitis) and otosyphilis (sudden sensorineural hearing loss), the CDC 2021 guideline uses the same intravenous penicillin neurosyphilis regimen, since these involve analogous sanctuary-site penetration challenges.

5. The Jarisch-Herxheimer Reaction: Mechanism and Management

Within two to eight hours of the first penicillin dose for syphilis, 50 to 75% of patients with primary or secondary syphilis experience the Jarisch-Herxheimer reaction (JHR) — a febrile, flu-like syndrome that can be alarming if not anticipated.

What it feels like:

• Fever rising to 39–40°C (102–104°F), often with chills and sweats
• Headache, myalgias (muscle aches), and malaise
• Temporary worsening or flaring of the syphilitic rash or other active lesions
• Onset within 2–8 hours; peak at 6–8 hours; resolution within 12–24 hours in most cases

What causes it: The JHR is not an allergic reaction to penicillin. It is caused by the rapid release of inflammatory mediators — particularly tumor necrosis factor-alpha (tumor necrosis factor-α), interleukin-6 (IL-6), and interleukin-8 (IL-8) — in response to the sudden killing of large numbers of spirochetes and the release of their cell-wall lipoproteins. These lipoproteins trigger an acute inflammatory cascade, essentially a brief, intense version of the same response that causes sepsis symptoms from gram-negative bacteria.

Management:

• Antipyretics (acetaminophen 650–1000 mg) provide symptomatic relief; the CDC 2021 guideline notes they have not been proven to prevent the reaction
• Adequate hydration
• Reassurance that this is expected and self-limiting
• The penicillin is continued. The CDC 2021 guideline describes the JHR as a reaction to treatment, not an allergic reaction to penicillin. Stopping treatment would leave the syphilis incompletely treated.

Groups requiring special caution:

• Pregnant women: The JHR can trigger uterine contractions and, rarely, premature labor or fetal distress. Under the CDC 2021 guideline, women treated in the second half of pregnancy are at risk of premature labor or fetal distress if treatment sets off the JHR, and the guideline advises them to seek obstetric care after treatment if they notice fever, contractions or fewer fetal movements.
• Neurosyphilis patients: Transient worsening of neurologic symptoms is possible.
• Cardiovascular syphilis patients: The acute inflammatory response can transiently worsen aortic valve regurgitation.

6. Alternatives for Penicillin Allergy

For non-pregnant patients with documented penicillin allergy who cannot undergo desensitization, the following alternatives are used, though with less supporting evidence than penicillin:

Doxycycline:

• Primary/secondary/early latent: 100 mg orally twice daily for 14 days (the CDC 2021 regimen for primary and secondary syphilis)
• Late latent: 100 mg orally twice daily for 28 days
• Advantages: oral, inexpensive, widely available
• Limitations: not used in pregnancy; requires patient adherence over 14–28 days vs. a single injection; data on efficacy, while supportive, derive largely from case series rather than randomized trials

Ceftriaxone:

• Primary and secondary syphilis: 1 g IM or intravenous daily for 10 days, per the CDC 2021 guideline, which states that the optimal dose and duration have not been defined
• Neurosyphilis: 1–2 g IM or intravenous daily for 10–14 days, which the CDC 2021 guideline lists from limited data as an alternative
• Advantages: parenteral (ensuring drug delivery), some central nervous system penetration, low cross-reactivity with penicillin (<2%)
• Limitations: requires daily injections or intravenous access; evidence is mainly case series; optimal dose and duration uncertain

Azithromycin: Previously used as an alternative but now no longer recommended — the CDC 2021 guideline states that “azithromycin should not be used as treatment for syphilis” because of documented treatment failures and macrolide resistance (the A2058G and A2059G mutations in 23S rRNA) now prevalent in many geographic areas including the US, UK, and Australia. A patient treated with azithromycin in an area with resistance has a high probability of treatment failure.

For pregnant women with penicillin allergy: The CDC 2021 guideline states that pregnant women with syphilis who report penicillin allergy be desensitized and treated with penicillin, the only therapy with documented efficacy for syphilis during pregnancy. Alternative regimens have not been proven to prevent congenital syphilis.

7. Treatment Failure and Retreatment

Treatment failure is defined as:

• Failure of RPR/VDRL titer to decline fourfold within 6 to 12 months of treatment for early syphilis
• A fourfold or greater rise in RPR/VDRL titer after initial decline (suggesting reinfection or reactivation)
• Persistence or development of new clinical signs of syphilis

When treatment failure is suspected, the usual steps are:

1. Reinfection is ruled out (the most common cause of rising titers — the patient has been exposed again)
2. Neurosyphilis is looked for with a lumbar puncture (even without neurologic symptoms)
3. If neurosyphilis is excluded, treatment is as for late latent syphilis (benzathine penicillin G 2.4 million units weekly ×3)
4. If the cerebrospinal fluid is abnormal, treatment is as for neurosyphilis (aqueous penicillin G intravenous)

True treatment failure with benzathine penicillin G (not reinfection, not the JHR, not the expected slow titer decline of late latent syphilis) is rare. Most apparent failures involve reinfection, incomplete initial regimens, or failure to achieve the fourfold titer definition due to the slow serologic response in late-stage disease.

8. Monitoring Treatment: RPR Titer Decline

After treatment, RPR or VDRL titers are checked at regular intervals. The goal is to confirm that the titer is declining, indicating successful treatment. This is the only objective measure of treatment efficacy available for syphilis.

Expected titer responses:

• Early syphilis (primary, secondary, early latent): the CDC 2021 guideline checks the titer at 6 and 12 months and treats a failure to fall fourfold within 12 months after treatment of primary or secondary syphilis as a possible sign of treatment failure; it also notes that 10–20% of people given the recommended therapy do not reach the fourfold fall within 12 months. Many patients achieve eightfold or greater decline within 12 months; some reach serofast status (stable low-positive titer, typically 1:1 to 1:4, that does not fully revert to negative)
• Late latent syphilis: Titer decline is slower; a fourfold decline within 12 to 24 months is considered adequate
• Neurosyphilis: the CDC 2021 guideline states that repeated cerebrospinal fluid examinations are unnecessary for people without HIV, or with HIV on antiretroviral therapy, who show serologic and clinical responses after treatment, because normalization of the serum RPR titer predicts normalization of the cerebrospinal fluid. In a study of 110 people treated for neurosyphilis, the serum RPR titer had normalized in 57% by 4 months, 85% by 7 months and 88% by 13 months (Marra 2008).

Serofast state: A small percentage of patients, especially those treated for late-stage disease, remain with a persistently positive non-treponemal test (usually at low titer, 1:1 to 1:4) after otherwise successful treatment. This does not represent ongoing infection — treponemal tests are positive for life, and some patients retain detectable but non-rising non-treponemal antibodies. The key distinction: in serofast status, the titer is stable and low; in treatment failure or reinfection, the titer rises fourfold or more.

Follow-up schedule in the CDC 2021 guideline: clinical and serologic evaluation at 6 and 12 months for early syphilis; at 6, 12, and 24 months for late latent; more frequently for patients with human immunodeficiency virus (HIV) co-infection.

Key Research Papers

1. Workowski KA, Bachmann LH, Chan PA, et al. Sexually transmitted infections treatment guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1–187. — PubMed PMID: 34292926
2. Stamm LV. Syphilis: antibiotic treatment and resistance. Epidemiol Infect. 2015;143(8):1567–1574. — PubMed PMID: 25358292
3. Lukehart SA, Hook EW 3rd, Baker-Zander SA, Collier AC, Critchlow CW, Handsfield HH. Invasion of the central nervous system by Treponema pallidum: implications for diagnosis and treatment. Ann Intern Med. 1988;109(11):855–862. — PubMed PMID: 3056164
4. Rolfs RT, Joesoef MR, Hendershot EF, et al. A randomized trial of enhanced therapy for early syphilis in patients with and without human immunodeficiency virus infection. N Engl J Med. 1997;337(5):307–314. — PubMed PMID: 9235493
5. Marra CM, Maxwell CL, Tantalo LC, et al. Normalization of serum rapid plasma reagin titer predicts normalization of cerebrospinal fluid and clinical abnormalities after treatment of neurosyphilis. Clin Infect Dis. 2008;47(7):893–899. — PubMed PMID: 18715154
6. Bolan GA, Sparling PF, Wasserheit JN. The emerging threat of untreatable gonococcal infection. N Engl J Med. 2012;366(6):485–487. — PubMed PMID: 22316442
7. Sexually Transmitted Disease Surveillance 2022. US Dept of Health and Human Services; 2023. — Search PubMed
8. Myint M, Bashiri H, Harrington RD, Marra CM. Relapse of secondary syphilis after benzathine penicillin G: molecular analysis. Sex Transm Dis. 2004;31(3):196–199. — PubMed PMID: 15076935
9. Ghanem KG, Ram S, Rice PA. The modern epidemic of syphilis. N Engl J Med. 2020;382(9):845–854. — PubMed PMID: 32101666
10. Edmondson DG, Hu B, Norris SJ. Long-term in vitro culture of the syphilis spirochete Treponema pallidum subsp. pallidum. mBio. 2018;9(3):e01153-18. — PubMed PMID: 29946052
11. Hook EW 3rd, Dionne JA, Workowski K, et al. One dose versus three doses of benzathine penicillin G in early syphilis. N Engl J Med. 2025;393(9):869–878. — PubMed PMID: 40902161

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