US drug regulators have affirmed a landmark study results showing that a popular HIV-fighting pill can also help healthy people avoid contracting the virus that causes Aids in the first place.
While the pill appears safe and effective for prevention, scientists stressed that it only works when taken on a daily basis.
The Food and Drug Administration (FDA) will hold a meeting tomorrow (NZ time) to discuss whether Truvada should be approved for people who are at risks of contracting HIV through sexual intercourse.
The agency's positive review suggests the daily pill will become the first drug approved to prevent HIV infection in high-risk patients.
FDA reviewers conclude that taking Truvada pre-emptively could spare patients ''infection with a serious and life-threatening illness that requires lifelong treatment''.
Despite the positive results, reviewers said that patients must be diligent about taking the pill every day.
Adherence to the medication was less than perfect in clinical trials, and reviewers said that patients in the real world may forget to take their medication even more than those in clinical studies.
First announced in 2010, Truvada's preventive ability was hailed as a breakthrough in the 30-year campaign against the AIDS epidemic.
A three-year study found that daily doses cut the risk of infection in healthy gay and bisexual men by 44 per cent, when accompanied by condoms and counseling.
Another study found that Truvada reduced infection by 75 per cent in heterosexual couples in which one partner was HIV infected and the other was not.
The FDA is not required to follow the advice of its panels, though it usually does.
An estimated 1.2 million Americans have HIV, which overwhelmingly affects men who have sex with other men, according to the Centers for Disease Control and Prevention.
HIV attacks the immune system and, unless treated with antiviral drugs, develops into Aids, a fatal condition in which the body cannot fight off foreign infections.
Because Truvada is already on the market to manage HIV, some doctors currently prescribe it as a preventive measure. FDA approval would allow the U.S. drugmaker Gilead Sciences to formally market its drug for the new use.
But support for FDA approval is not unanimous. Some researchers stress that condoms remain the best weapon against Aids, and a prevention pill is not the chemical equivalent.
''We know that if the person doesn't take the medication every day they will not be protected,'' said Dr Rodney Wright, director of HIV programs at Montefiore Medical Center in New York.
''So the concern is that there may not be adequate adherence to provide protection in the general population.''
Wright, who is also chairman of the Aids Health Foundation, added that some upcoming medications may be more effective at preventing infection than Truvada.
Researchers also worry about Truvada's mixed success rate in preventing infection among women.
Last year a study in women was stopped early after researchers found that women taking the drug were more likely to become infected than those taking placebo.
Researchers speculated that women may need a higher dose of the drug to prevent infection. They also said the disappointing results may have resulted from women not taking the pills consistently.