A similar recommendation is already in place for gay men and heterosexual couples at high risk of catching HIV, the virus that causes AIDS.
The latest advice was triggered by the results of a study done in Thailand that showed the AIDS drug tenofovir protected many drug users. Volunteers who took the daily pill were about 50 percent less likely to become infected than those given a dummy pill.
"This study completes the story" telling how HIV drugs can protect people at highest risk of infection, said Dr. Jonathan Mermin, director of AIDS prevention for the Centers for Disease Control and Prevention.
The research by the CDC and the Thailand government was published online Wednesday by the journal Lancet.
Based on the findings, the CDC recommended that doctors consider prescribing tenofovir to those who inject drugs. It blocks the virus from making copies and spreading through the body. In the U.S., tenofovir is included in an AIDS drug called Truvada.
HIV infections in drug users is a bigger problem worldwide, where they account for about 1 in 10 new cases each year and the vast majority of infections in some places in Eastern Europe and central Asia. In the U.S., they represent about 1 in 13 new cases.
How many people already take the drug in the U.S. to protect against infection isn't known; the CDC only began recommending it for that purpose the last two years. And health officials acknowledge it's not clear how many of the nation's 1 million injection drug users would have the money or insurance to pay for it.
The only approved version in the U.S. is Truvada, made by Gilead Sciences Inc. It costs more than $14,000 a year. In Thailand, the tenofovir used in the study costs about $360.
Truvada came on the market in 2004 to treat people who already have the AIDS virus. Since then, six studies have been done in different high risk groups to see if it could prevent infections. Two failed but health officials believe those results were skewed because many participants did not take the drugs faithfully.
The latest study was the first in people who inject drugs. It looked at about 2,400 uninfected patients at 17 drug treatment clinics in Bangkok, Thailand's capital and largest city. Half the participants received daily tenofovir and half got a dummy pill; all got condoms and counseling to discourage the spread of HIV.
Over the next four years, risky behaviors—like sharing needles and having multiple sex partners—fell at equal rates in both groups. There were 17 HIV infections in the tenofovir group compared to 33 in the group given dummy pills. That meant the treated group's risk was reduced by 49 percent.
"It does not completely eliminate your risk of HIV," said Jennifer Kates, director of global health and HIV policy for the Kaiser Family Foundation, a research and policy organization in Washington, D.C.
But she voiced relief about the findings, saying: "We were waiting for the results of this trial."
International health officials celebrated the findings as an important advance.
"Piece by piece, scientific advances are paving the way to the end of the AIDS epidemic," Michel Sidibe, executive director of UNAIDS, said in a statement.
HIV information: http://www.cdc.gov/hiv