A drug used to treat genital herpes can also slow progression of the AIDS virus among co-infected patients, doctors report on Monday.
Most people infected with HIV are also coincidentally infected with type 2 herpes simplex virus, or HSV2.
Previous lab research has shown that using an anti-HSV2 drug called aciclovir reduces the amount of AIDS virus in the blood.
What was unknown, though, was whether this also had an impact on the spread of HIV in the body.
The answer, according to a study published by The Lancet, is that aciclovir does have a braking effect that is modest but could become a useful option for doctors.
It could buy time before a patient has to be given powerful drugs to combat HIV.
Jairam Lingappa of the University of Washington led a trial in 14 sites in southern and eastern Africa, recruiting 3,381 heterosexual volunteers who were co-infected with HSV2 and HIV-1, the main strain of the human immunodeficiency virus that causes AIDS.
Half of the group were assigned to aciclovir, taking twice-daily doses of the drug, or to a dummy lookalike pill, and were followed for up to 24 months.
The key question was whether the volunteers' count of CD4 cells, the immune cells that are destroyed by HIV, fell below 200 per microlitre by the end of the trial.
On this score, aciclovir reduced the risk by 16 percent. Of the volunteers on aciclovir, 284 fell below the 200 CD4 count, whereas the number among the placebo group was 324.
Using another yardstick, the researchers also found that, among volunteers whose immune system was in better shape, aciclovir also reduced by 19 percent the risk of a CD4 count falling below 350 cells per microlitre.
The findings are important, says Lingappa, because they suggest another weapon could be added to the skimpy pharmaceutical armoury for treating HIV.
Recent evidence has shown that people with HIV have a better chance of survival if they are given antiretrovirals at an earlier stage of infection.
As a result, the World Health Organisation (WHO) has revised its guidelines for treating HIV in poor countries.
It recommends that these daily drugs be administered when the CD4 cell count falls to 350 per microlitre, rather than wait for the CD4 tally to hit a lower level, when the immune system can become badly damaged.
These recommendations have a wide-ranging impact, not least because they mean cash-strapped countries have to find extra money to treat people sooner than before.
So a drug that can slow progression towards this mark will ease the strain on health budgets. It would also stave off the time when a patient has to take a treatment that can have toxic side effects.
"While the HIV disease-ameliorating effect we have observed is modest, it could add one more tool to help people with HIV infection stay healthy for longer," Lingappa said. Further investigation is needed, he added.