Doxycycline is the best-supported alternative antibiotic for treating syphilis in HIV-positive patients during penicillin shortages.
Treatment Recommendations by Syphilis Stage
Early Syphilis (Primary, Secondary, or Early Latent)
For HIV-positive patients when penicillin is unavailable, use doxycycline 100 mg orally twice daily for 14 days 1, 2. This is the most extensively studied alternative regimen with acceptable efficacy data in HIV-infected populations 3, 4.
- A 2026 systematic review and network meta-analysis found doxycycline to be the best-supported alternative to benzathine penicillin, despite low-quality evidence 5
- A 2019 natural experiment in HIV-infected patients showed comparable serological response rates: 72% with doxycycline versus 70% with penicillin (no statistically significant difference) 3
- A 2012 study demonstrated similar time to serological response among HIV-infected patients treated with doxycycline, ceftriaxone, or benzathine penicillin 4
Late Latent Syphilis
Use doxycycline 100 mg orally twice daily for 28 days 1, 2. While this regimen has not been as extensively evaluated in HIV-infected persons, it remains the recommended alternative when penicillin cannot be used.
Neurosyphilis
Ceftriaxone 2 g daily (IV or IM) for 10-14 days is the preferred alternative 6, 2, 6, 2.
- A 2026 multicenter study in people with HIV showed ceftriaxone had comparable early serological response to IV penicillin G (71.8% vs 74.4%, no significant difference) 7
- Critical caveat: Cross-reactivity between ceftriaxone and penicillin exists; if true penicillin allergy is suspected, skin testing should be performed if available, followed by desensitization if necessary 6
Alternative Agents: Important Limitations
Azithromycin - NOT RECOMMENDED
Despite some evidence suggesting a single 2-gram oral dose may have efficacy for early syphilis, azithromycin should NOT be used due to widespread macrolide resistance in T. pallidum and documented treatment failures in the United States 2, 8.
Ceftriaxone for Early Syphilis
While 1 g daily IV/IM for 10 days showed comparable efficacy to benzathine penicillin in one randomized trial 8, the evidence remains limited. Guidelines state this has not been sufficiently evaluated in HIV-infected persons to warrant first-line alternative status 2.
Amoxicillin Plus Probenecid
A 2023 study suggested short-term (5-11 days) amoxicillin 3 g/day with probenecid might be effective 9, but sample sizes were very small and there is insufficient evidence to recommend this regimen 8.
Critical Management Considerations
Close clinical and serologic monitoring is mandatory with any alternative regimen 2. HIV-infected patients should be evaluated at 3,6,9,12, and 24 months after therapy 6.
When to Consider Desensitization Instead
If compliance with follow-up cannot be ensured, penicillin desensitization is preferred over alternative antibiotics 1, 2. This is particularly important for neurosyphilis, where desensitization is the preferred approach for penicillin-allergic patients 2.
Practical Algorithm
- Determine syphilis stage (early vs late latent vs neurosyphilis)
- If early or late latent syphilis: Use doxycycline (14 days for early, 28 days for late latent)
- If neurosyphilis: Use ceftriaxone 2 g daily for 10-14 days (check for penicillin allergy history first)
- If true penicillin allergy with neurosyphilis: Pursue desensitization if feasible
- Ensure rigorous follow-up: Schedule visits at 3,6,9,12, and 24 months with repeat nontreponemal testing
Common Pitfalls to Avoid
- Do not use azithromycin - resistance is widespread in the US
- Do not assume ceftriaxone is safe in penicillin-allergic patients - cross-reactivity exists
- Do not use suboptimal doxycycline duration - 14 days for early syphilis, 28 days for late latent
- Do not skip close follow-up - alternative regimens require more intensive monitoring than penicillin
- Do not forget that HIV-infected patients may have higher rates of treatment failure - this makes follow-up even more critical 6