Doxycycline Post‑Exposure Prophylaxis Effectively Reduces Syphilis Risk and Serial Negative Serology Excludes Primary Infection

Doxycycline Post‑Exposure Prophylaxis (PEP)

In sexually active adults, a single dose of 200 mg doxycycline taken within 72 hours after condomless sex, followed by 100 mg daily for three additional days, provides extended antimicrobial coverage and is recommended by the CDC for bacterial STI prevention. 1
The CDC’s standard doxy‑PEP regimen limits the total daily dose to 200 mg and advises a maximum of one 200 mg dose per 24 hours. 4
Ongoing counseling about continued doxy‑PEP use is advised for individuals anticipating future high‑risk sexual encounters, as this strategy further lowers bacterial STI acquisition risk. [1][4]

Protected vaginal intercourse carries a minimal risk of syphilis transmission according to CDC data. 1
Unprotected oral sex transmits syphilis at a lower efficiency than unprotected anal or vaginal intercourse, based on CDC estimates. 1

Friction‑induced erosion or fissure resulting from prolonged physical activity without proper hygiene is a plausible non‑STI cause of genital skin lesions. 2
Fixed drug eruption or contact dermatitis should be considered in the differential diagnosis of genital eruptions unrelated to infectious etiologies. 2
Herpes simplex virus infection, although typically recurrent and painful, remains a differential diagnosis for genital ulcerative lesions. 3

The CDC recommends routine STI screening (including syphilis, chlamydia, gonorrhea, and HIV) every 3–6 months for individuals with ongoing sexual exposure risk. [3][2]
Continuation of regular STI screening every 3–6 months is appropriate even after a negative syphilis work‑up, to maintain surveillance in the context of ongoing sexual activity. 1