A 35-year-old woman with a history of penicillin anaphylaxis presents with a 6-month history of maculopapular rash on the trunk and palms, mucous patches in the mouth, and generalized lymphadenopathy. RPR titre is 1:64 with positive FTA-ABS. She is not pregnant. What is the drug of choice for secondary syphilis in this patient?

Explanation

## Secondary Syphilis in Penicillin-Allergic Patient **Key Point:** Doxycycline 100 mg orally twice daily for 14 days is the preferred alternative for secondary syphilis in non-pregnant, penicillin-allergic patients. ### Clinical Context This patient has confirmed secondary syphilis: - Constitutional symptoms (rash, mucous patches, lymphadenopathy) - High RPR titre (1:64) indicating active spirochetemia - Positive FTA-ABS (treponemal-specific test, confirms diagnosis) - **Penicillin anaphylaxis** — absolute contraindication to beta-lactams - **Not pregnant** — allows use of tetracyclines ### Treatment Algorithm for Syphilis by Stage and Allergy Status ```mermaid flowchart TD A["Confirmed Syphilis<br/>(Primary, Secondary, or Early Latent)"]:::outcome A --> B{"Penicillin allergy?"}:::decision B -->|"No allergy"| C["Benzathine Penicillin G<br/>2.4 MU IM × 1 dose<br/>(or 3 doses for late latent)"]:::action B -->|"Yes, non-anaphylaxis"| D{"Pregnant?"}:::decision B -->|"Anaphylaxis"| E{"Pregnant?"}:::decision D -->|"No"| F["Doxycycline 100 mg BD<br/>× 14 days"]:::action D -->|"Yes"| G["Ceftriaxone 1 g IM daily<br/>× 10 days"]:::action E -->|"No"| F E -->|"Yes"| H["Penicillin desensitization<br/>then Benzathine PG"]:::urgent F --> I["Adequate for primary,<br/>secondary, early latent"]:::outcome G --> I ``` ### Comparative Efficacy in Secondary Syphilis | Agent | Dose & Duration | Efficacy | CNS Penetration | Pregnancy Safe | First-Line Status | |-------|-----------------|----------|-----------------|-----------------|-------------------| | **Benzathine PG** | 2.4 MU IM × 1 | >95% | Adequate | Yes | **Gold standard** | | **Doxycycline** | 100 mg BD × 14 days | ~95% | Good | **No** | **Alt. if allergy, non-pregnant** | | **Ceftriaxone** | 1 g IM daily × 10 days | ~95% | Excellent | Yes | Alt. if allergy (any pregnancy) | | **Azithromycin** | 500 mg daily × 7 days | ~90% | Poor | No | **Not recommended** (resistance) | **High-Yield:** In non-pregnant, penicillin-allergic patients: - **Doxycycline is preferred** for primary, secondary, and early latent syphilis - Achieves adequate serum and CNS levels - 14-day course ensures treponemicidal effect - Cost-effective and oral (good compliance if educated) **Clinical Pearl:** Penicillin anaphylaxis is a **true contraindication** to all beta-lactams (including cephalosporins, though cross-reactivity is ~1–3%). Doxycycline avoids this risk entirely and is highly effective. **Mnemonic:** **DAD-14** = Doxycycline, Allergy, non-pregnant → 14 days. ### Why NOT Ceftriaxone Here? - Although safe in anaphylaxis-prone patients, it is **not preferred** for secondary syphilis in non-pregnant individuals - Doxycycline is equally effective, cheaper, and oral - Reserve ceftriaxone for pregnant, penicillin-allergic patients (cannot use tetracyclines) [cite:CDC STI Treatment Guidelines 2021; Harrison 21e Ch 207]