A reformulated vaginal microbicide gel, originally designed to prevent HIV transmission in women, has proved safe and well-tolerated in HIV-negative men and women using it rectally. With the antiretroviral tenofovir as an active agent, the gel used a lowered amount of glycerin in hopes of creating a product more amenable to rectal use. Sixty-five study participants were divided into four groups using different products over the course of a week: the reformulated gel, a placebo gel without tenofovir, or a gel including the spermicide nonoxynol-9. A fourth group only received the study's procedures and tests, which included physical and rectal exams, and no gel. With no major difference in side effect rates between the groups using gels, 80 percent reported minor side effects and 18 percent moderate ones. Ninety-four percent used the gels daily as directed. Eighty-seven percent of the repurposed gel group said they would likely use it again, compared with 93 percent of the placebo gel group and 63 percent of the nonoxynol-9 group.