Gonorrhoea. It is quite easy to catch a gonorrhoea throat infection by performing oral sex on a man who has it. He will have a discharge from the urethral opening (the hole at the end of the penis), and this discharge will be teeming with gonorrhoea bacteria. Unfortunately, the discharge is not always very noticeable, so neither partner may be aware that he has the infection. Alternatively, he may have noticed a discharge previously, but may have assumed that the problem had cured itself. The message is that you should not perform oral sex on a man who has a discharge, or has recently had one that was not treated.

Warts. It is not a good idea to perform oral sex on a man who has genital warts. These may not be obvious, so check the margin of the head of the penis ('glans') under the foreskin, and just inside the urethral opening. Although the risk is not great, a partner can develop a wart on the roof of their mouth as a result of oral sex. The wart may take weeks or even months to appear. If you develop one, go to your local genitourinary medicine clinic.

Similarly, it is possible for a partner to develop a wart on their lips from performing oral sex on a woman who has genital warts. If this happens to you, a genitourinary medicine clinic is the place to go.

HIV. It is possible to catch HIV by oral sex. The US Government Centers for Disease Control and Prevention has done research among men in San Francisco who were recently infected with HIV. In 7% of cases, the virus was probably caught during oral sex. Although the research was among homosexual men, a woman could catch it from a man similarly.

The body fluids of a person who has HIV (including semen and the natural fluids that lubricate the vagina) will contain the virus. So HIV can enter the mouth of the person performing oral sex, which might lead to infection if he or she happens to have any sores or cracks in the mouth. The risk is likely to be greater for someone performing oral sex on a man than the other way round. Withdrawing the penis before the man has ejaculated would lessen the risk; however, when a man is sexually aroused, small amounts of semen leak out of the urethra before ejaculation, so infection could still occur. Although the risk is low, it is not absolutely safe, and oral sex with someone who might be HIV-positive cannot be described as 'safe sex'. Some experts say you should avoid mouth-to-genital contact unless you are very sure that your partner is HIV-negative. Otherwise, avoid brushing your teeth beforehand, because this could open up cracks in the gum that would make it easier for the virus to enter, and don’t do it if you have a sore throat.

Syphilis is uncommon in the UK, but seems to be increasing. A recent outbreak was reported in the journal Communicable Disease and Public Health in 2001. Most of the people infected were homosexual men who had oral sex without using a condom.

Swallowing semen during oral sex is not harmful. The main risk of infection is in the mouth as described above. Swallowing semen doesn't have any extra risk because the stomach contains acid that destroys HIV.

Rimming (licking around the anus) is not recommended. The lower bowel contains many bacteria and viruses that could enter the mouth, even if the anal area is washed beforehand.