Decline in chemsex over time in three English clinics

In the first study to track chemsex over time, the number of men having chemsex fell by two-thirds between 2015 and 2018, Janey Sewell and colleagues at University College London report in the June issue of the International Journal of Drug Policy. Chemsex has been defined by researchers as the use of recreational drugs such as mephedrone, GHB/GBL and crystal meth to enhance sex.

Data come from a prospective cohort study, Attitudes to and Understanding Risk of Acquisition of HIV over time (AURAH2), which recruited HIV-negative or undiagnosed gay and bisexual men at three London and Brighton sexual health clinics: 56 Dean Street, the Mortimer Market Centre and the Claude Nicol Centre.

The data from the study may be unlikely to represent national trends, as these three clinics have a reputation for providing high quality services to gay men engaging in chemsex, with substance use and sexual health being addressed holistically. The clinics are likely to both attract more patients who engage in chemsex and also be better at supporting them than some other services.



The use of recreational drugs such as mephedrone, GHB/GBL and crystal meth before or during sex.

longitudinal study

A study in which information is collected on people over several weeks, months or years. People may be followed forward in time (a prospective study), or information may be collected on past events (a retrospective study).


When using a diagnostic test, the probability that a person who does have a medical condition will receive the correct test result (i.e. positive). 

prospective study

A type of longitudinal study in which people join the study and information is then collected on them for several weeks, months or years. 

statistical significance

Statistical tests are used to judge whether the results of a study could be due to chance and would not be confirmed if the study was repeated. If result is probably not due to chance, the results are ‘statistically significant’. 

The participants were recruited between November 2014 and April 2016, completed an initial questionnaire in the clinic, and were then invited to complete quarterly online questionnaires for up to three years. Data for this analysis come from 622 men who completed at least one online questionnaire. As with all studies, ongoing engagement with the study declined over time (458 completed the questionnaire 12 months after recruitment, 376 did so after 24 months, and 72 did after 36 months).

Participants mostly identified as gay (95%), were university educated (77%) and employed (89%). There was a wide range of ages, with a median of 34 years. While 84% were white, 43% were born outside the UK.

In the first online questionnaire, 32% said that they had “used drugs before or during sex (chemsex)” in the past three months. The most common drug was mephedrone (25%), followed by GHB/GBL (20%) and crystal methamphetamine (11%).

There was a steady decline in each quarterly survey: in the last survey, three years after first recruitment, 11% reported any chemsex, 9.7% reported mephedrone use in chemsex, and 8.3% reported GHB/GBL in chemsex. Each of these falls was statistically significant.

However, there was no decline in use of crystal meth over time.

A plausible explanation of this apparent fall could have been that men engaging in chemsex would be more likely to drop out of the study, leaving ‘less risky’ men to provide data. However, the researchers found that there was no association between chemsex and not subsequently completing questionnaires. In addition, a sensitivity analysis which only included those men who completed a survey in the last six months of the study found a similar decline in chemsex over time.

Instead, the researchers offer three possible explanations for the fall in chemsex:

  • The quality and effectiveness of the chemsex support provided in the three clinics.
  • Repeatedly completing questionnaires that encourage reflection on behaviour may have helped the study participants become more conscious of the consequences of their choices, which could have led to behaviour change.
  • The phenomenon described by statisticians as ‘regression to the mean’. A variable that is extreme on its first measurement will tend to be closer to the centre of the distribution for a later measurement. In other words, men may have been recruited to the study at a time of particularly high-risk behaviour, which would then tend to decline with time.

There were also some changes in sexual behaviour over time. Men were a little more likely to have condomless sex with more than one partner as the study progressed, but it was less likely that this sex involved a partner of unknown HIV status or group sex. There were larger falls in bacterial STIs (from 26% to 10%).

The researchers note that in contrast to a previous report from St George’s Hospital in south London, their longitudinal study did not find an association between chemsex and a new diagnosis of HIV. There were relatively few seroconversions (15) among study participants.


Sewell J et al. Changes in chemsex and sexual behaviour over time, among a cohort of MSM in London and Brighton: Findings from the AURAH2 study. International Journal of Drug Policy 68: 54-61, 2019. (Full text freely available).