The study, published online on Oct. 4 by the British journal BMJ, pooled data from studies done in nine countries. It concluded that making methadone available reduced H.I.V. risk by 54 percent.
Many countries, including Russia, have large H.I.V. epidemics among addicts but nonetheless outlaw methadone and buprenorphine treatment for political, religious or other reasons.
Methadone itself does not affect the virus; scientists believe it works because addicts on treatment become better able to stop sharing needles and selling sex for drugs. They are also better able to stay on antiretroviral drugs, which lowers the chance they will infect others. The authors speculated that opiate-substitution therapy was so effective because addicts who are motivated enough to seek treatment are also smart about protecting themselves in other ways.
Contaminated needles cause 5 percent to 10 percent of all the world’s H.I.V. infections, the study estimated. The problem is worst in Eastern Europe and in Central and Southeast Asia on the fringes of opium-growing areas.
Although more people have sex than inject drugs, the risk of catching H.I.V. is 1 in 125 from a syringe, about 1 in 122 from anal sex and less than 1 in 2,000 from vaginal sex, according to an editorial published with the study.