What are the recommended antibiotics for treating syphilis?

Antibiotics for Syphilis

Benzathine penicillin G remains the definitive first-line treatment for all stages of syphilis, with stage-specific dosing protocols that directly prevent progression to irreversible cardiovascular and neurological complications.

Primary Treatment Recommendations by Stage

Early Syphilis (Primary, Secondary, Early Latent)

• Benzathine penicillin G 2.4 million units IM as a single dose 1, 2
• This single injection is highly effective for preventing progression and achieving serologic cure
• For HIV-infected patients, the same single-dose regimen is recommended, though some specialists advocate for three weekly doses (benefit remains unproven) 1

Late Latent Syphilis or Syphilis of Unknown Duration

• Benzathine penicillin G 7.2 million units total: administered as 2.4 million units IM weekly for 3 consecutive weeks 3, 4
• This extended regimen prevents late complications including cardiovascular and gummatous disease
• CSF examination should be performed if neurologic/ophthalmic symptoms, treatment failure, or HIV infection with late latent disease is present 4

Neurosyphilis

• Aqueous crystalline penicillin G 18-24 million units IV daily (administered as 3-4 million units IV every 4 hours or continuous infusion) for 10-14 days 2
• Alternative: Procaine penicillin 2.4 million units IM daily PLUS probenecid 500 mg orally four times daily for 10-14 days 2
• Consider adding benzathine penicillin 2.4 million units IM weekly for up to 3 weeks after completion to provide comparable total therapy duration 2

Tertiary Syphilis (Cardiovascular/Gummatous)

• Benzathine penicillin G 7.2 million units total: 2.4 million units IM weekly for 3 weeks 3
• CSF examination mandatory before initiating therapy to exclude neurosyphilis 3, 2
• Manage in consultation with infectious disease specialist 2

Penicillin Allergy Alternatives

For Early Syphilis (Non-Pregnant)

• Doxycycline 100 mg orally twice daily for 14 days 3, 5
• Alternative: Tetracycline 500 mg orally four times daily for 14 days 3
• Ceftriaxone 1 g IV/IM daily for 10 days is a reasonable alternative based on randomized trial data 6
• Doxycycline has reassuring retrospective data supporting efficacy 6

Critical caveat: Azithromycin (2 g single oral dose) should NOT be used in the United States due to widespread macrolide resistance and documented treatment failures, despite some international efficacy data 1, 6, 7

For Late Latent Syphilis (Non-Pregnant)

• Doxycycline 100 mg orally twice daily for 28 days 3
• Alternative: Tetracycline 500 mg orally four times daily for 28 days 3

For Pregnant Patients with Penicillin Allergy

• Penicillin desensitization is mandatory, followed by penicillin therapy 3, 4
• No alternative antibiotics are acceptable in pregnancy—desensitization is the only option 3

Special Populations

HIV-Infected Patients

• Use identical penicillin regimens as HIV-negative patients 1
• Enhanced therapy (benzathine penicillin plus amoxicillin/probenecid) does NOT improve outcomes and is not recommended 1
• Closer serologic follow-up is warranted to detect treatment failure 1
• Limited data suggest no benefit to multiple doses over single-dose therapy for early syphilis in terms of serologic outcomes 6

Pediatric Dosing

• Early latent: Benzathine penicillin G 50,000 units/kg IM (max 2.4 million units) as single dose 3, 4
• Late latent: Benzathine penicillin G 50,000 units/kg IM weekly for 3 weeks (total 150,000 units/kg, max 7.2 million units total) 3, 4

Critical Follow-Up Parameters

Early Syphilis

• Quantitative nontreponemal tests at 3 and 6 months 5
• Treatment failure indicators: persistent/recurrent symptoms, sustained fourfold titer increase, or failure of titers to decline fourfold by 3 months 5
• Re-treatment with three weekly benzathine penicillin doses if failure occurs 5

Latent Syphilis

• Serologic testing at 6,12, and 24 months 3, 4
• Re-evaluate for neurosyphilis if: titers increase fourfold, initially high titer (≥1:32) fails to decline fourfold within 12-24 months, or new symptoms develop 3, 4

Neurosyphilis

• Repeat CSF examination every 6 months until cell count normalizes (if initially elevated) 2
• CSF leukocyte count is the most sensitive measure of treatment effectiveness 2
• Consider retreatment if cell count hasn't decreased after 6 months or CSF parameters not normal after 2 years 2

Common Pitfalls to Avoid

1. Missed doses in late syphilis treatment: If a weekly dose is missed, an interval of 10-14 days between doses may be acceptable before restarting the sequence, but this is NEVER acceptable in pregnant patients—they must restart the full course 4

2. Assuming azithromycin is equivalent: Despite meta-analysis data showing comparable efficacy 7, widespread resistance in the US makes this unacceptable 1, 6

3. Inadequate screening frequency: HIV-infected persons and men who have sex with men require screening every 3-6 months, not annually 1

4. Overlooking neurosyphilis: Any patient with neurologic/ophthalmic symptoms, tertiary disease manifestations, treatment failure, or HIV with late latent disease requires CSF examination 4, 2

5. Using alternatives in pregnancy: Only penicillin after desensitization is acceptable—no exceptions 3, 4

The evidence consistently demonstrates that penicillin remains unmatched in preventing the devastating morbidity and mortality of untreated syphilis, particularly neurosyphilis and cardiovascular complications that develop in late-stage disease 3, 1, 2, 6.