The finding comes from a new look at the study data that led the FDA to approve Truvada for HIV prevention. The drug is used as part of a strategy called pre-exposure prophylaxis (or PrEP). PrEP is extremely effective for uninfected men at greatest risk of HIV who take Truvada once each day.
Men who do this have almost total protection against HIV infection. Those who miss three doses a week still get 96% protection, say Robert M. Grant, MD, MPH, associate director of the University of California, San Francisco Center for AIDS Research, and colleagues.
"For potential users of PrEP, the message is that daily dosing is still our recommendation, because daily use can become part of people's routine," Grant says. "People also can know there is some forgiveness. If you try to take it daily and miss a dose, it will be OK. But we do not recommend non-daily dosing."
The findings are based on a study of men who have sex with men. The findings do not apply to women. Concentrations of Truvada's active ingredient are much higher in the rectum than in the vagina.
At least two clinical trials are under way to see whether some people might benefit from taking Truvada fewer than seven days a week. Until those results are in, Grant warns people using the anti-HIV pill not to deliberately miss doses.
PrEP has been controversial, as consistent condom use is extremely effective for preventing HIV infection. But after decades of promoting condom use, HIV continues to spread in the U.S. and elsewhere.
"We are hearing from HIV-negative partners of HIV-positive people who want an added sense of security," Grant says. "And we hear from people who cannot use condoms consistently and finally can admit that to themselves. They know condoms are a first line of defense, but people are not using them consistently."
PrEP gets widespread support from community groups working to prevent AIDS, as well as from AIDS clinicians like Jeffrey Lennox, MD, co-director of the Emory Center for AIDS Research, Clinical Research Core.
"In general, PrEP is a great concept because it allows people to take an extra step to protect themselves against HIV, particularly those having difficulty negotiating 100% condom use. I support the concept," Lennox says.
However, Lennox notes that there are practical problems with PrEP. One is that Truvada is expensive. State programs to provide AIDS drugs to people who can't afford them already have waiting lists of people already infected with HIV.
"In many areas of the U.S. where the AIDS epidemic is burgeoning, PrEP is not likely to be used at all," Lennox says. "The greatest areas of increasing HIV infection are in the Southeast, where public funding for PrEP is essentially nonexistent."