The evidence for this is clearest in studies of couples in which one person is infected with HIV and other not i.e. "discordant couples". In a study of discordant couples in Europe among 123 couples who reported consistent condom use, none of the uninfected partners became infected. In contrast, among the 122 couples who used condoms inconsistently, 12 of the uninfected partners became infected.

In the laboratory, latex condoms are very effective at blocking transmission of HIV because pores in latex condoms are too small to allow the virus to pass through. However outside of the laboratory condoms are less effective because people do not always use condoms properly.

What are condoms made off?

Condoms are made of latex (rubber) or polyurethane. Latex condoms are most effective against HIV and are commonly used. Lubrication on condoms also varies. Some condoms are not lubricated at all, some are lubricated with a silicone substance & some condoms have a water-based lubricant. The lubrication on condoms aims to make the condoms easier to put on and more comfortable to use.

Polyurethane condoms are thinner than latex condoms and so they increase sensitivity and are more agreeable in feel and appearance to some users. They are also useful to those people who are allergic to latex. But a disadvantage is that they are much more likely to break than latex condoms.