Earlier studies have shown that Gilead's Truvada (tenofovir-emtricitabine), which was approved in the U.S for "pre-exposure prophylaxis" (PrEP) of the AIDS-causing HIV, can reduce risks of infection.

The drug performed well in highly controlled clinical trials that compared its effects to that of a placebo but some people were concerned about the effectiveness of Truvada when used in real world setting.

It was not also clear if an increase in the risky behavior of those who take the HIV prevention pill would offset the benefits of the medication.  Now, two new studies provide evidence that the pill indeed works.

In a study published in Clinical Infectious Diseases, researchers involved 657 people in San Francisco area, most of whom were gay and bisexual men between 20 and 68 years old who took Truvada daily for a period of three years from 2012 and 2015.

During this period, none of the participants had new HIV infection albeit half of them contracted new sexually transmitted infections (STIs). The high incidence of STI suggests that the participants still engaged in high risk practices.

In another study published in The Lancet, researchers focused on gay men and found that those who started on PrEP had significantly fewer new HIV infection compared with their counterparts who waited a year for the pill.

New HIV infection only affected two per 100 people per year in those who used PrEP and nine per 100 people per year in those whose access to the drug was delayed.  The researchers likewise said that two thirds of those on PrEP who were diagnosed with HIV likely had the infection when they began the study.

"The impressive reduction in HIV incidence in people taking PrEP, without a measurable increase in other sexually transmitted infections, is reassuring for clinical, community, and public health stakeholders," said Sheena McCormack, from the University College London, who took part in The Lancet study.

Truvada is a combination therapy composed of two medicines in one pill. The drug works by making it harder for the virus to multiply in the body.

Once HIV enters the body, it multiplies inside CD4 cells or T cells that are crucial for fighting infections. The new viruses go into the blood and then infect other CD4 cells.

Truvada works by making it more difficult for HIV-1 to multiply by blocking the enzyme known as reverse transcriptase. By preventing HIV-1 from spreading, the drug lowers the viral load and decreases the amount of HIV in the blood. Truvada also help increase the number of CD4 cells when it is used alongside another anti-HIV 1 medication. 

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