Nonoccupational post-exposure prophylaxis (NPEP) regimens containing tenofovir were better tolerated and more likely to be completed than those containing zidovudine, according to a study conducted at the Fenway Community Health center in Boston and reported in the April 1 issue of the Journal of Acquired Immune Deficiency Syndromes (JAIDS). NPEP is the use of antiretroviral drugs following high-risk sexual exposure in order to prevent HIV transmission.

There has been one large cohort study in humans showing that the use of zidovudine—most often combined with lamivudine, coformulated as Combivir—may prevent transmission of HIV by as much as 75 percent in health care workers who start taking the drugs within 72 hours after being exposed to HIV in the course of their jobs, often through needle sticks, and who continue to take the drugs for four weeks. Although there has been no similar study of PEP for sexual exposures, PEP is believed to be at least somewhat effective and is offered in many emergency rooms and clinics when people have had unprotected anal or vaginal sex with a partner they suspect is HIV positive. However, one of the major downsides of PEP, and a factor that can impact the efficacy of the strategy, is that the drugs’ side effects often cause more than half of the people who are prescribed PEP to stop taking them before finishing their course of treatment.

Kenneth Mayer, MD, of the Fenway clinic and Brown University in Providence, Rhode Island, and his colleagues compared 112 people who were prescribed NPEP that included tenofovir with 122 people who were prescribed NPEP that included zidovudine. The majority of the study participants were white, male and had been potentially exposed to HIV through unprotected anal sex, either as the insertive or receptive partner.

Mayer’s team found that 73 percent of those who received tenofovir with emtricitabine—often used together in the coformulated tablet Truvada—and 87 percent of those who received tenofovir with lamivudine completed the full four weeks of treatment. Only 42 percent of those taking zidovudine with lamivudine and 39 percent taking zidovudine plus two additional drugs completed their course of treatment.

As many as 37 percent of those taking tenofovir had diarrhea, and 47 percent complained of bloating and stomach pain; however, few people discontinued treatment with tenofovir due to side effects. That compares with more than half of those taking a regimen with zidovudine complaining of nausea and vomiting and a significant proportion stopping treatment because of the side effects’ severity.

People taking NPEP reduced instances of unprotected sex by as much as 80 percent during the month they were taking the therapy. About 5 percent of the people who took NPEP, however, did so more than once—because they engaged in unprotected sex again after completing NPEP—and the authors suggest that risk-reduction counseling and other behavioral interventions should be provided along with NPEP.

Mayer’s team also points out that the majority of the participants reported being high on alcohol, drugs or both when they had unprotected sex. The team concludes that behavioral interventions offered to people who need NPEP should also focus on substance use and other factors that are correlated with unprotected sex.