The risk of transmitting HIV by oral sex is much less clear.

The Health Protection Agency, which monitors HIV in the UK, estimates that about 1 to 3% of all sexual transmissions of HIV are due to oral sex. And a review of all the scientific literature on the risk of HIV transmission from oral sex concluded that this was very unlikely, but not zero.

It is widely accepted that the risk of passing on HIV from oral sex is much smaller than the risk during unprotected anal or vaginal sex. Giving oral sex to a man is higher risk than receiving it.

The following are thought to increase the risk of passing on HIV during oral sex:

• the person being given oral sex having a very high viral load or an untreated sexually transmitted infection
• if an HIV-positive man ejaculates in the mouth of the person giving oral sex
• if an HIV-positive man has cuts, sores or rashes on his penis
• bleeding gums, sores or wounds in the mouth of the person giving oral sex
• a sore throat, inflammation or untreated infection in the mouth of the person giving oral sex.

The risk of transmission from an HIV-positive woman to someone giving her oral sex is thought to be extremely small. However, the levels of HIV in vaginal fluid vary. They are likely to be highest around the time of your period, when HIV-bearing cells shed from the cervix are most likely to be found in vaginal fluid, along with blood. Oral sex will therefore be more risky for the person giving oral sex around the time of menstruation and if they have bleeding gums, sores or wounds, or a sore throat, inflammation or untreated infection in the mouth. You can reduce the risk further by using a dental dam (a sheet of latex) or using a piece of latex cut from a condom, as protection.