The probability of gay men in England having unprotected
anal intercourse is strongly associated with the number of recreational drugs
they have taken at the time, according to a study presented to the 20th
International AIDS Conference (AIDS 2014) in Melbourne this week. Although the finding will
confirm many people’s common sense beliefs, it has not previously been demonstrated
in UK research.
Previous studies have found associations between men using drugs at
least once in the past few months and men having unprotected sex at least once during that period – but have not analysed their interaction during a single event.
The data come from the Sigma Panel, a group of gay and
bisexual men who took part in regular web surveys about their sexual health and
HIV prevention in 2011 and 2012. On a number of occasions, the surveys asked
detailed questions about the last sexual encounter that respondents had had. A
total of 2142 men provided information about 6742 sexual encounters – including
where it took place, what kind of relationship they had with the other man,
which sexual acts were involved, how pleasurable it was, whether any drugs were
used and whether the respondent felt in control of what he did.
Whereas there was a 25% probability of unprotected sex when
no substances were used, this rose to 30% when one substance had been taken,
50% with three substances and a 75% probability of unprotected sex when more
than five substances had been taken.
Poppers, GHB and crystal methamphetamine were the drugs most
frequently associated with unprotected sex.
For this analysis, GJ Melendez-Torres of the University of
Oxford also used an innovative statistical technique, called multilevel class
analysis, in order to better characterise the ‘personality’ of sexual
The statistical analysis identified three distinct types of
encounters. This is not a definitive
description (it was in part a demonstration that the statistical process can be
applied to this kind of data), but did indicate that some types of encounters
are more likely than others to involve unprotected sex. The researcher said
that a larger data set might provide a more nuanced description, potentially with
more than three classes of encounters.
encounters that typically occurred with a regular, steady partner, at home,
with little substance use, there was a 41% probability of unprotected sex. This
class accounted for 31% of all sexual encounters.
sexual events that usually occurred with non-steady or anonymous partners, with
little substance use, there was a much lower, 23% probability of unprotected
sex. This class accounted for the majority (62%) of all encounters.
third class of event was distinct – it only amounted to 7% of encounters but
was associated with a 53% probability of unprotected anal intercourse. It was
characterised by use of multiple substances with non-steady or anonymous
The results suggest there could be value in targeting
interventions that help men with the last class of encounters, which only
amount to a minority of sexual events but have a high probability of
unprotected sex. The findings also confirm that a very large number of
unprotected sex encounters occur within primary relationships, although men do
not necessarily have accurate information about whether they have the same HIV
status as their partner.
In general, men were less
likely to have sex without a condom when they were with an anonymous partner,
with a partner they had not discussed HIV status with, with a partner they
knew to have a different HIV status, when they were in a cruising location, or
when they were in a sex-on-premises venue. More unprotected sex happens in