Drugs and sex. Taking drugs and having risky sex: forever intertwined in our imagination and in people’s actual behaviour. In the field of sexual health, this is mostly taken for granted. Doctors, nurses, counsellors, educators and researchers alike know that there is a strong likelihood that someone who regularly takes drugs is also likely to engage in ‘risky’ sex, and the two activities are likely to be indicative of someone ‘at high risk’ in other aspects of their lives. Mostly, we understand this through the prism of vulnerability: individuals who take socially disapproved risks very often are thought of as victims, whether due to social disadvantage, or mental or emotional impairment. The fundamental question we need to address, though, is what the available evidence tells us about these ‘taken-for-granted’ links. The linkages between drugs, sex, and disadvantage raise important questions. Is drug use necessarily problematic? Does drug use cause sexual risk-taking? Are both drug use and sexual risk-taking indicative of underlying emotional or other problems? There are, of course, broader social harms associated with drug use in general and with specific drugs in particular, as alluded to by Mullens et al. published in a previous issue. The negative consequences of specific drugs vary according to their particular effects, both biological and psychological, and depending on their level of use; and these also vary according to individual circumstances. In the case of methamphetamine, for example, its long half life means that users often remain ‘up’ (and then ‘down’) for several days, potentially disrupting their lives quite substantially. Nonetheless, these negative consequences do not automatically mean there will be negative consequences in users’ sexual behaviour, and the evidence for this is complex and sometimes contradictory. The link between drug use, sex and the risk of HIV or other sexually transmissible infections among gay men has been well documented, and common sense suggests that if drugs impair judgement and distort perception then this is likely to lead to poor decisions about sexual (and other) behaviour. That this happens to some people is, of course, undeniable, but most research in this field is based on cross-sectional data, with an association between the two behaviours over a given time period. A statistical association does not necessarily indicate cause and effect. Regardless of the time period involved, this only tells us that those who use drugs are often the same people who take risks sexually. Some studies are based on event-level data, indicating an association between drug use and sexual risk behaviour on a specific occasion: on occasions when gay men in these studies used drugs, they were also less likely to use condoms. However, not all studies using event-level data have found such an association. Australian data from two separate cohort studies of gay men, one HIV-negative and one HIV-positive, found otherwise. The most recent encounter involving unprotected anal intercourse (UAI) was compared with the most recent encounter involving condom use: among men who engaged in UAI, drug use did not distinguish these two events. These Australian data suggest that while drug use is associated with sexual risk behaviour in general, it may not be a primary driver for specific decisions not to use condoms. Many, perhaps most, of those who use drugs remain quite capable of making decisions about sexual risk. Cohort studies have also identified a strong association between drug use and sexual risk events, and between drug use and HIV seroconversion. In most cases this is explored in one direction: does drug use predict subsequent sexual risk, or subsequent HIV seroconversion? In a recent analysis of some Australian data, however, it was found that this association was bi-directional: sexual risk behaviour predicted subsequent uptake of drug use. So, while cohort data provide a stronger case for the association between drug use and sexual risk-taking, they do not necessarily prove a specific cause-and-effect relationship. Indeed, it is probably not possible to obtain such direct evidence, but that may be the wrong question anyway. Perhaps what we really need is to understand the motivations for both behaviours – sexual risk-taking and drug use. What is the link between them and why are some individuals especially likely to engage in both? The connection between drug use and sexual risk taking has applied to all different types of drugs, both licit and illicit, including alcohol, as it has to drugs in general. Nonetheless, some drugs appear to be particularly implicated in sexual risk-taking over the past decade. Amyl nitrite was originally singled out as a potential and specific risk factor for HIV CSIRO PUBLISHING Editorial