Shares in Gilead were given a boost on news that US regulators have agreed to conduct a speedy review of its application to market HIV drug Truvada for prevention of the disease.

The US Food and Drug Administration has granted the supplemental New Drug Application a priority review, which means that in just six months' time Truvada (emtricitabine/tenofovir disoproxil fumarate) could well become the first available drug on the market for preventing HIV infection in uninfected adults.

Following its approval in 2004 Truvada has become the top-selling antiretroviral for HIV treatment in the US, and pulls in annual sales of $2.9 billion a year. A green light on the prevention side could boost the its sales substantially, even by as much as $1 billion, according to some reports.

The sNDA is based on data from two large placebo-controlled trials of Truvada sponsored by the US National Institutes of Health and the University of Washington, but several other clinical studies support the use of the drug in this indication, Gilead said.

In one study, published in 2010 in the New England Journal of Medicine, Truvada cut the risk of gay men contracting HIV by an average 44% and by more than 70% in those taking it the most frequently, raising hopes for an effective new weapon against the disease.