US health authorities are recommending the daily use of anti-retroviral medication to prevent HIV infection for high-risk groups.

The new guidelines by the US Centers for Disease Control and Prevention unveiled this week recommends taking Truvada, a pill containing tenofovir and emtricitabine, based on studies that showed the drug can help reduce infection rates by more than 90 percent when taken every day.

Truvada is the only anti-retroviral drug approved for the prevention method known as pre-exposure prophylaxis, or PrEP.

The Food and Drug Administration greenlighted the drug, made by Gilead Sciences, in 2012.

Up to 500,000 Americans could benefit from this treatment, compared to just 10,000 people who currently take such medication. It is covered by insurance.

The CDC's new directives especially concern at-risk populations such as gay men who have sexual relations without a condom, heterosexuals with high-risk partners such as users of injected drugs, bisexual men who have unprotected sex and anyone who has regular sexual relations with partners who are known to be infected with HIV.

All users of injected drugs or people who share needles are also considered at risk.

The CDC says HIV testing is required before starting the therapy, to be renewed at least every three months.

The therapy aims to turn the tide against a trend that has seen about 50,000 people get infected with HIV each year for the past 20 years, despite condom use campaigns.

But the CDC guidelines also "underscore (the) importance of counseling about adherence and HIV risk reduction, including encouraging condom use for additional protection."

Indeed, the health watchdog recognizes that nothing guarantees that the recommendations for the antiretroviral drugs will be followed by those people at substantial risk for HIV, stressing the urgency for action as the use of condoms decreases in gay men.

A CDC study published in November showed a 20 percent increase of unprotected sex between gay men from 2005 to 2011.

Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, expressed support for the CDC guidelines but warned that "people might think if they take the drug, they won't use condoms."

"We need all prevention modalities we can get and... the pill is highly effective," Fauci told AFP.

But some groups criticized the initiative.

The AIDS Healthcare Foundation, the biggest provider of HIV/AIDS medical care in the US, said the CDC was "ill-advised" to promote PrEP as HIV prevention.

"This is a position I fear the CDC will come to regret," said AHF president Michael Weinstein.

"The CDC has abandoned a science-driven, public health approach to disease prevention -- a move that will likely have catastrophic consequences in the fight against AIDS in this country."

Weinstein warned that despite the requirements for risk-reduction counseling and condom use while taking Truvada, "the government-sanctioned widespread deployment of PrEP will be accompanied with a shift to condom-less sex."

He said the decision could also increase the occurrence of sexually transmitted diseases such as syphilis.

Weinstein urged the CDC to focus on HIV testing and the treatment of the estimated 20 percent of the 1.3 million people living with HIV in the United States who have not yet been tested for the disease.