Treatment Regimen for Syphilis
Benzathine penicillin G remains the definitive treatment for all stages of syphilis, with dosing and duration varying by stage of disease. Here's the algorithmic approach:
Primary, Secondary, and Early Latent Syphilis
Treat with benzathine penicillin G 2.4 million units IM as a single dose 1, 2. This applies to both HIV-negative and HIV-positive patients, though HIV-infected individuals require more intensive follow-up 2.
- Early latent syphilis is defined as infection acquired within the preceding year, documented by seroconversion, unequivocal symptoms of primary/secondary syphilis, or a sex partner with documented early syphilis 1
- This single-dose regimen has over 40 years of clinical experience supporting its efficacy 3
Late Latent Syphilis or Syphilis of Unknown Duration
Administer benzathine penicillin G 7.2 million units total: three doses of 2.4 million units IM given at weekly intervals 1, 2.
Critical caveat: If a patient misses a dose during weekly therapy, an interval of 10-14 days between doses may be acceptable before restarting the sequence, though 7-9 days is more optimal pharmacologically 4. However, missed doses are never acceptable for pregnant patients 1.
Neurosyphilis
Use IV aqueous crystalline penicillin G 18-24 million units daily (administered as 3-4 million units IV every 4 hours or by continuous infusion) for 10-14 days 5.
Alternative regimen: Procaine penicillin 2.4 million units IM once daily plus probenecid 500 mg orally four times daily for 10-14 days 5. Note: Do not use probenecid in sulfa-allergic patients 5.
Some experts recommend following neurosyphilis treatment with benzathine penicillin G 2.4 million units IM weekly for up to 3 weeks to provide comparable total treatment duration 5.
When to Suspect Neurosyphilis (Requires CSF Examination)
- Neurologic or ophthalmic signs/symptoms
- Evidence of tertiary syphilis (aortitis, gumma, iritis)
- Treatment failure
- HIV infection with late latent syphilis or syphilis of unknown duration 1
- HIV-infected patients with CD4 count ≤350 cells/mL and/or RPR titer ≥1:32 (though CSF examination in asymptomatic patients hasn't shown improved outcomes) 2
Penicillin-Allergic Patients
For non-pregnant patients with primary, secondary, or early latent syphilis:
Doxycycline 100 mg orally twice daily for 14 days is the best-supported alternative 6, 7. Recent network meta-analysis (2026) found doxycycline to be the best-supported alternative despite low-quality evidence 6.
For late latent syphilis in penicillin-allergic patients:
- Doxycycline 100 mg orally twice daily for 28 days 5
- This has not been sufficiently evaluated in HIV-infected persons 5
For neurosyphilis in penicillin-allergic patients:
- Penicillin desensitization is preferred 5
- Ceftriaxone 2 g IV daily for 10-14 days may be an acceptable alternative, though data are limited 5
Critical warning: Azithromycin should not be used due to documented molecular resistance and clinical treatment failures 5, 8, 7.
For pregnant patients: Penicillin allergy requires desensitization followed by penicillin treatment—no alternatives are acceptable 1.
HIV-Infected Patients: Special Considerations
Treatment regimens are the same as for HIV-negative patients 2. There is no added benefit to enhanced antimicrobial therapy in HIV-infected persons 4.
However, follow-up must be more intensive:
- Evaluate clinically and serologically at 3,6,9,12, and 24 months after therapy for early syphilis 1, 2
- For late latent syphilis: evaluate at 6,12,18, and 24 months 1
Treatment failure criteria: Signs/symptoms persist or recur, OR sustained fourfold increase in nontreponemal test titer 2. When this occurs, perform CSF examination and retreat 2.
Consider CSF examination and retreatment if nontreponemal titers don't decrease fourfold within 6-12 months 1, 2.
Follow-Up for All Patients
For early syphilis (HIV-negative): Clinical and serologic evaluation at 6 and 12 months 1
For late latent syphilis: Serologic monitoring at 6,12,18, and 24 months to ensure at least fourfold decline in titer 1
Important: Approximately 15-20% of patients remain "serofast" (low unchanging titers, usually <1:8) after successful treatment—this does not represent treatment failure 5, 4. Reinfection is diagnosed by at least a fourfold increase above the serofast baseline 5.
Pregnancy
All pregnant women must be screened for syphilis at the first prenatal visit 1. High-risk populations require repeat testing in the third trimester at 28 weeks and at delivery 9.
Penicillin is the only acceptable treatment during pregnancy—penicillin-allergic pregnant women must undergo desensitization 9, 1.