Chemsex and drug interactions

What chemsex is

Sexual activity, usually between men who have sex with men, under the influence of specific drugs — most commonly crystal methamphetamine, GHB/GBL and mephedrone — typically over extended sessions of many hours or days, often with multiple partners.

It is a recognised public health concern in several European and North American cities. It is not a moral category, and the people involved are not a separate population; effective responses treat it as a health issue with specific, addressable risks.

The drugs and their specific dangers

GHB / GBL. The most dangerous by a wide margin in this context. The gap between an active dose and an overdose dose is very small — sometimes a matter of half a millilitre — and varies with the batch's concentration, which is usually unknown. It has a steep dose-response curve, causing sudden unconsciousness ("going under") with no gradual warning.

Combining GHB with alcohol, benzodiazepines, opioids or ketamine dramatically increases the risk of respiratory depression and death. Redosing before the previous dose has worn off is the most common route to overdose, because effects can take up to 45 minutes and impatience is common.

GHB is physically dependence-forming with regular use, and withdrawal is a medical emergency comparable to severe alcohol withdrawal — delirium, seizures and autonomic instability, with documented deaths. Do not stop daily use abruptly without medical support.

Crystal methamphetamine. Long duration, intense stimulation, marked reduction in judgement and risk perception, and suppression of hunger, thirst and sleep across multi-day sessions. Associated with dehydration, hyperthermia, cardiac arrhythmia, psychosis with repeated use, severe dental damage, and profound depressive crash afterwards. Injecting ("slamming") introduces bloodborne virus and injection-site infection risk.

Mephedrone. Shorter acting, frequent redosing, cardiovascular strain, and a marked comedown.

Ketamine. Dissociation impairs the ability to assess or communicate consent, and eliminates pain feedback — a substantial injury risk during anal sex and fisting. Long-term heavy use causes ketamine-induced ulcerative cystitis, sometimes requiring bladder removal.

Poppers (alkyl nitrites). Smooth muscle relaxation, used to ease anal penetration. Absolutely contraindicated with PDE5 inhibitors such as sildenafil and tadalafil: the combination causes severe, potentially fatal hypotension. This interaction kills people and is not widely enough known. Also associated with maculopathy causing visual disturbance, and dangerous in glaucoma or with severe anaemia.

Interactions with HIV medication

This is a specific, under-communicated danger. Ritonavir and cobicistat, used as pharmacokinetic boosters in some antiretroviral regimens, inhibit CYP3A4 and can dramatically raise blood levels of MDMA, methamphetamine, ketamine and GHB. Deaths from MDMA in people taking ritonavir have been documented since the 1990s.

Anyone on antiretroviral therapy who uses these drugs should tell their HIV clinician. This conversation is confidential, HIV services are experienced with it, and the interaction information can be lifesaving. Antiretroviral therapy should never be stopped to accommodate drug use — that risks resistance and treatment failure.

Infection risk

Extended sessions with multiple partners, reduced barrier use, mucosal trauma from prolonged activity and reduced pain feedback, and shared injecting equipment together produce a distinctive risk profile:

• Elevated HIV and hepatitis C transmission. The sexually transmitted hepatitis C epidemic among men who have sex with men is closely associated with chemsex.
• High rates of bacterial STIs, frequently at multiple sites.
• Shigella and other enteric infections.

Practical measures: PrEP, hepatitis A and B vaccination, three-site testing every three months or more often, doxy-PEP where indicated, never sharing injecting or snorting equipment, single-use lubricant rather than shared pots, and gloves changed between partners.

Consent

Consent given while heavily intoxicated is compromised, and severe intoxication removes capacity entirely. Extended sessions with multiple people and multiple substances make this genuinely difficult territory, and sexual assault within chemsex settings is under-reported and under-supported.

Practical protections: agree limits before using anything; tell someone outside the session where you are and when you will check in; do not attend somewhere you cannot leave; keep your phone charged and keep control of your own drugs and dosing.

Harm reduction

• Never dose GHB for someone else, and never accept a pre-mixed dose. Use a syringe to measure, write down the time of every dose, and set a timer.
• Never mix GHB with alcohol.
• Stay hydrated with water, not only alcohol; eat something; try to sleep.
• Never leave someone unconscious to sleep it off. Recovery position, stay with them, and call emergency services for unresponsiveness, snoring or gurgling breathing, blue lips, seizures, or vomiting while unconscious. Paramedics are there to treat, not to report.
• Plan the comedown: the days after a session carry a real suicide risk, particularly with methamphetamine. Arrange contact with someone, and know a helpline number in advance.

Getting support

Specialist chemsex services exist in many cities and are non-judgemental, confidential and frequently attached to sexual health clinics. In the UK, 56 Dean Street and Antidote (London Friend) are the best-known; comparable services operate elsewhere. Support is available for reducing, for stopping, and for continuing more safely.

Sources

• Bourne A et al. "The Chemsex Study: drug use in sexual settings among gay and bisexual men." Sigma Research, LSHTM, 2014.
• Maxwell S, Shahmanesh M, Gafos M. "Chemsex behaviours among men who have sex with men: a systematic review." International Journal of Drug Policy, 2019.
• Liverpool HIV Drug Interactions database (hiv-druginteractions.org) — antiretroviral and recreational drug interactions.
• Bracchi M et al. "Increasing use of 'party drugs' in people living with HIV on antiretrovirals: a concern for patient safety." AIDS, 2015.
• van Amsterdam J, Brunt TM, van den Brink W. "The adverse health effects of synthetic cannabinoids and GHB." Reviews on GHB toxicity and dependence.
• Hagan H et al. "Incidence of sexually transmitted hepatitis C virus infection in HIV-positive men who have sex with men." AIDS, 2015.
• BASHH and UKHSA. Chemsex clinical guidance and harm reduction resources, current editions.

Last reviewed: August 2026.