Capacity, intoxication and consent

Capacity

Consent requires the capacity to give it. A person lacks capacity when they cannot understand what is proposed, retain that information long enough to decide, weigh it, and communicate a decision. This is the four-part test in the Mental Capacity Act 2005 (England and Wales) and is mirrored in most comparable jurisdictions.

Capacity is decision-specific and time-specific. Someone may lack capacity for a complex financial decision and have capacity for a sexual one, and capacity may be present in the morning and absent in the evening. A diagnosis — dementia, intellectual disability, mental illness — does not itself remove capacity, and blanket assumptions in either direction are unlawful under the Act's presumption of capacity and supported decision-making duty.

Intoxication

There is no blood alcohol threshold that separates consent from assault, and no jurisdiction has legislated one, because intoxication is a continuum. What the law and clinical guidance consistently identify as beyond capacity:

• Slurring badly, unable to walk unaided, or repeatedly losing balance
• Unable to follow or hold a conversation
• Vomiting, drifting in and out of awareness, or falling asleep
• Not knowing where they are, what time it is, or who is present
• Unconscious or asleep — never able to consent, without exception

England and Wales case law is explicit: R v Bree (2007) held that a person can consent while drunk, but that consent is absent if they have lost the capacity to choose through drink. The practical test in the same judgment: capacity, not sobriety.

Two further points that matter in practice.

Alcohol-induced amnesia. Blackout means memory encoding has failed while behaviour continues. Someone in a blackout can talk, walk and appear to participate while having no capacity to weigh a decision and no subsequent memory. Appearing functional is not evidence of capacity.

Being intoxicated yourself is not a defence. It does not reduce responsibility for assessing the other person's state, in law or in ethics.

The practical standard

If you are wondering whether they are too drunk, they are too drunk. This is not a legal test but it is the only reliable one available in the moment, and it costs nothing to apply. Wait. If they had capacity, they will still be interested tomorrow.

Drug-facilitated assault

Alcohol is by a wide margin the substance most often involved in drug-facilitated sexual assault — far more common than covert drugging, though covert drugging is real. Substances used include benzodiazepines, GHB/GBL and ketamine. GHB has a narrow margin between recreational and dangerous dose and interacts severely with alcohol.

Warning signs of covert drugging: sudden disproportionate intoxication relative to what was consumed, unexplained confusion, memory gaps, waking with no memory, unexplained physical symptoms. Response: stay with someone you trust, seek medical attention early, and request toxicology screening — most substances clear rapidly, GHB within roughly 6–12 hours in blood and up to about 12 hours in urine, so testing is time-critical.

Spiking by injection has been reported in the UK and elsewhere; evidence on prevalence is limited, but any unexplained puncture mark with sudden symptoms warrants urgent medical assessment.

Sleep, unconsciousness and advance agreement

A sleeping or unconscious person cannot consent. Advance agreement to be woken with sex is a real arrangement between some partners, and it is only meaningful where it was negotiated in detail while both were awake and sober, is bounded to specified acts, and either partner can revoke it at any time. Canadian law is clear that advance consent to sexual activity during unconsciousness is not legally valid consent (R v J.A., 2011). Where such an arrangement exists between partners, the person acting still carries the responsibility to stop at any sign of distress.

Other capacity factors

Severe sleep deprivation, acute mental health crisis, delirium, hypoglycaemia, head injury, and some medications (sedatives, some anti-epileptics, opioids) can all impair capacity. Power imbalance — employer, clinician, teacher, therapist, custodial officer, immigration sponsor — does not remove capacity but can make consent invalid because it is not freely given; most professional regulators prohibit sexual contact with current patients or clients outright.

Sources

• Mental Capacity Act 2005 (England and Wales), sections 1–3.
• R v Bree [2007] EWCA Crim 804.
• R v J.A., 2011 SCC 28.
• Sexual Offences Act 2003 (England and Wales), sections 74–76.
• Wetherill RR, Fromme K. "Alcohol-induced blackouts: a review of recent clinical research." Alcoholism: Clinical and Experimental Research, 2016.
• Anderson LJ et al. "A systematic review of drug-facilitated sexual assault." Journal of Forensic and Legal Medicine, 2017.
• Faculty of Forensic and Legal Medicine. Guidance on drug-facilitated sexual assault, current edition.

Last reviewed: August 2026.