Taking 100 mg of the antibiotic doxycycline daily significantly reduced the risk of acquiring another sexually transmitted infection (STI), according to a pilot study among 30 HIV-positive gay men in Los Angeles.
The study, published in Sexually Transmitted Diseases, also noted that widespread use of antibiotics as prophylaxis may lead to the development of drug-resistant syphilis strains. The study authors therefore cautioned that "daily doxycycline prophylaxis against syphilis would not be an appropriate prevention tool for most HIV-[positive gay men] but only for a relatively small, yet epidemiologically important population, where its use may outweigh the theoretical resistance risks."
Prior to being included in the study, participants had had at least two syphilis infections since their HIV diagnosis, which was a median of eight to 12 years before the study. Sixty percent of patients were Latino and only one person was African American. All were tested for Neisseria gonorrhoeae, Chlamydia trachomatis and syphilis at each study visit (at 12-week intervals) and completed a behavioral questionnaire each time.
Study participants were randomized to two arms of 15 each: One group received financial incentives for remaining STI-free, while the other took the doxycycline prophylaxis for 36 weeks, with a final study visit at 48 weeks. Only 31% of those in the antibiotic arm had blood levels of the drug at each study visit indicating daily adherence (1,000 ng/mL), but 62% had such levels at two of three visits while on doxycycline.
Even at these less-than-optimal drug concentrations, only six of the 21 new STI infections (nine syphilis, 12 gonorrhea or chlamydia) that occurred during the study were among those in the prophylaxis group (P = .02). While half of the STIs in the doxycycline arm (two syphilis, one gonorrhea) were among participants whose blood levels of the drug indicated daily adherence, that infection could have occurred at any point during the prior three months, while drug level measurements only indicate adherence during the four to five days before the visit.
Self-reported behavior did not differ significantly between the groups, with over 60% reporting condomless sex since the previous study visit, and 20% reporting methamphetamine use during that period.
Given the small size, but promising outcome, of this study, the study authors concluded that, "A randomized clinical trial should be conducted to confirm and extend these findings."