Treating intravenous drug users with antiviral drugs may reduce their chances of HIV infection, according to a new study published Wednesday in the British medical journal The Lancet.

The Bangkok Tenofovir Study was done in Bangkok, Thailand, from 2005-2013. It was run by the Centers for Disease Control and Prevention in collaboration with the Thailand Ministry of Public Health.

Researchers recruited more than 24,000 people at 17 sites. Half took the pill tenofovir - an antiretroviral drug - daily, while the other half got a placebo. Participants were followed for about four years. Researchers found those taking the drug cut their chances of infection by 49% almost in half - approximately 49%.

This is not the first study to prove that PrEP - pre-exposure prophylaxis - is effective in reducing HIV transmission in high risk groups. Studies have already been done in heterosexuals as well as gay and bisexual men. But according to the CDC, this is the first study of its kind to prove effective in reducing rates among those who inject drugs.

“This study completes the picture of PrEP efficacy for all major HIV risk groups," said Dr. Michael Martin, chief of clinical research for the Thailand Ministry of Public Health-CDC collaboration. "We now know that pre-exposure prophylaxis can be a potentially vital option for HIV prevention in people at very high risk for infection, whether through sexual transmission or injecting drug use.”

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said, "The real important advance here is the proof of concept that in yet another high risk group - IV-drug users - pre-exposure prophylaxis can work if the individuals adhere to taking the drug."

The study found that the reduction rate for those who took their daily dose routinely was even higher - 74%.

“Adherence was a key factor determining efficacy in our trial among people who inject drugs, as it has been in previous PrEP trials examining sexual transmission,” said Martin. “These results underscore the importance of helping people using pre-exposure prophylaxis achieve effective levels of adherence.”

It can often be difficult to directly attribute the antiviral drug's protective effect on people who get infected by injecting drugs, said Salim Karim, professor and director of the Centre for the AIDS Programme of Research in South Africa (CAPRISA) at the University of KwaZulu-Natal in Durban, South Africa. That's because, he says, some IV drug users engage in risky behavior such as prostitution to support their drug habit.

“Even though questions remain about the extent to which PrEP can be effective in preventing either of the routes of transmission in this group, the overall result is that daily tenofovir does reduce HIV transmission in injecting drug users," Karim said. "The introduction of PrEP for HIV prevention in injecting drug users should be considered as an additional component to accompany other proven prevention strategies like needle exchange programmes, methadone programmes, promotion of safer sex and injecting practices, condoms, and HIV counseling and testing.”

UNAIDS, the Joint United Nations Programme on HIV/AIDS, called the study results a complement to other PrEP trials released over the past few years.

“Piece by piece, scientific advances are paving the way to the end of the AIDS epidemic,” said Michel Sidibé, UNAIDS executive director. “The full potential of antiretroviral therapy in keeping people alive and well and in preventing new HIV infections is becoming apparent. The results of this study are important, and if used effectively in HIV programming, could have a significant impact in protecting people who inject drugs from becoming infected with HIV.”

So far, all evidence suggests PrEP is safe and effective for reducing the risk of HIV infection in high-risk populations, and there have been no concerns regarding its use daily, the CDC says.