The more a person drinks the more likely they are to have unprotected sex, according to new research.
Experts found that for every slight increase in alcohol consumption, a person's willingness to engage in risky sex got stronger.
This held true even when other factors, such as sensation-seeking or a general tendency to risky behaviour, were taken into account.
Experts from the University of Toronto analysed the results of 12 studies involving men and women which looked at the relationship between alcohol and unsafe sex.
After pooling the results, they found alcohol consumption affects decision-making, and this impact rises with the amount of alcohol consumed.
Every increase in blood alcohol level of 0.1 mg/ml led to a 5% increase in the intention to engage in unprotected sex.
The UK legal limit for drivers is 80mg of alcohol per 100ml of blood.
Writing in the journal Addiction, the team concluded: "Alcohol use is an independent risk factor for intentions to engage in unprotected sex, and as risky sex intentions have been shown to be linked to actual risk behaviour, the role of alcohol consumption in the transmission of HIV and other sexually transmitted infections may be of public health importance."
Dr Jurgen Rehm, lead author on the study, said: "Drinking has a causal effect on the likelihood to engage in unsafe sex, and thus should be included as a major factor in preventive efforts for HIV.
"This result also helps explain why people at risk often show this behaviour despite better knowledge: alcohol is influencing their decision processes."