Doctors and researchers aren't sure how many people have been infected with HIV through oral sex. Some think hardly anybody has been infected with HIV from oral sex, but other people think that as many as 3% of infections are due to oral sex. In late 2008 researchers looked at all the available evidence and calculated that the risk of contracting HIV from oral sex was very low, but that it wasn't zero.

It is known that oral sex involves less risk than unprotected anal sex or unprotected vaginal sex.

The likelihood that HIV is transmitted from an HIV-positive person to an HIV-negative person depends on the type of contact involved. HIV is most easily transmitted by unprotected anal sex (that is, without condoms), unprotected vaginal sex, sharing injecting equipment, and from mother to baby. It also depends on the viral load of the person with HIV, and the dental health of the person performing oral sex.

Oral sex has been shown to be a less risky activity than these practices, but is definitely not risk free. It’s also worth remembering that other sexually transmitted infections, such as syphilis, herpes and gonorrhoea, can be quite easily transmitted through oral sex.

A number of studies have been published on the risks of HIV transmission from oral sex. One American study reported that of 122 gay men with HIV, 8% reported oral sex as their only risk activity. But this study doesn't have much credibility. The numbers of people were very small. Plus, some of the men in this study who initially said oral sex was their only risk activity, later said they'd had unprotected anal sex. 

It's worth noting that one study didn’t find any cases of oral transmission of HIV over a ten-year period in over 100 couples, where one partner was HIV-positive and the other HIV-negative.

Another study from the US involving gay men found the risk to be effectively zero, but didn't exclude the possibility of there being a risk.

For the 2008 report, researchers reviewed all the studies looking at the risk of HIV transmission from oral sex. They found that the risk was very low, but not zero.

If you have HIV, there is a higher risk of passing on HIV through someone performing oral sex on you, if you also have an untreated sexually transmitted infection. If you don't have HIV and you are performing oral sex on someone who does have HIV, you are at more risk of being infected if you have cuts, sores or abrasions in your mouth or on your gums. There is also more of a risk if you have an infection, including sexually transmitted ones, in your throat or mouth which is causing inflammation.

Having a high viral load in the blood may also mean that viral load is high in the semen. Although there is good evidence that men who have an undetectable viral load in their blood usually have an undetectable viral load in their semen, this isn't always the case. Factors like untreated sexually transmitted infections can cause viral load in semen to increase. Therefore, most doctors believe you cannot assume that having an undetectable viral load means you're uninfectious. However, the risk of HIV transmission during oral sex if a person has an undetectable viral load is extremely low.

The levels of HIV in vaginal fluid vary. They are likely to be highest around the time of menstruation (having 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 around the time of menstruation.

There are several ways to reduce the risks of oral sex. Naturally, some will be more acceptable than others to different individuals, so you must make your own decisions about the level of risk you find acceptable. If you would like to discuss these issues, ask to see a health adviser, or other health professional, at your treatment centre or sexual health clinic. Many of the strategies below will also provide protection against other sexually transmitted infections:

• You may decide that the risks of oral sex are low enough for you to continue your regular behaviour.

• You may prefer not to have oral sex because you do not wish to take even a low risk of HIV transmission.

• You may decide to reduce the number of partners with whom you have oral sex.

• You may decide to have oral sex with barriers such as condoms for men or dental dams (latex squares) for women.

• If you don't have HIV, you may decide only to have insertive oral sex (someone giving you oral sex) as this is safer than receptive oral sex (giving someone else oral sex).

• You may decide not to ejaculate into your partner’s mouth or not to have someone ejaculate into your mouth.

• You may decide to avoid oral sex during menstrual periods.

• Look after your mouth. The likelihood of becoming infected through giving oral sex increases if someone has bleeding gums, ulcers, cuts or sores in the mouth. Don’t brush your teeth or floss just before oral sex.

• Have regular sexual health screening. This will identify if you have any sexually transmitted infections, which may increase the likelihood of you transmitting HIV to a negative partner, and reduce the likelihood of you contracting HIV if you are HIV-negative.

For more information on oral sex and sexual transmission of HIV, you may find the information in our publications HIV & sex or HIV transmission & testing helpful. Online versions of these are freely available in the resources section of our website: www.aidsmap.com/resources.