Drug-injecting addicts who took a daily antiretroviral pill were half as likely to become infected with H.I.V. as those who did not, a major new study has found, providing the final piece of evidence that such treatments can prevent AIDS in every group at risk. Earlier clinical trials showed that the therapy can sharply reduce the risk of H.I.V. transmission from mother to child, and in gay and bisexual men and heterosexuals.

“This provides the totality of the evidence that the drugs used to treat the infection are also very effective at preventing it,” said Dr. Salim S. Abdool Karim, a prominent South African AIDS researcher who wrote a commentary in The Lancet, which published the new study on Wednesday.

The accumulating evidence from clinical trials means antiretroviral drugs are increasingly seen as another in the arsenal of weapons to prevent AIDS, along with condoms, abstinence and fidelity; early antiretroviral treatment; male circumcision in Africa; microbicide gels; and other options.

The formal results of the study, which involved 2,400 drug users in Thailand, showed that taking tenofovir pills — a therapy known as pre-exposure prophylaxis, or PrEP — reduced infections by 49 percent. Addicts who took the pills regularly, based on measures of tenofovir in their blood, did much better: They were 74 percent less likely to become infected.