UK Driving Limits: Alcohol vs Medical Cannabis

In the United Kingdom, the law draws a sharp distinction between alcohol and cannabis when it comes to driving. On the surface, both have specified legal limits. But the practical and legal realities could not be more different — particularly for the tens of thousands of medical cannabis patients navigating this regulatory landscape.

The Legal Limits

For alcohol, England and Wales operate a two-tier system. The lower limit of 35 micrograms of alcohol per 100 millilitres of breath applies in most circumstances, while a higher limit of 22 micrograms per 100 millilitres of breath exists for certain categories of drivers ( [GOV.UK, drink-drive limits]). Scotland unified its limit to 22 micrograms across the board in 2014. The blood alcohol concentration (BAC) equivalents are approximately 0.08% and 0.05% respectively.

For cannabis, the limit is 2 micrograms of delta-9-tetrahydrocannabinol (THC) per litre of blood ( [GOV.UK, drug-drive law]). This is an exceptionally low threshold. To put it in context, frequent users may retain residual THC in their bloodstream at levels exceeding this limit for days after their last consumption, long after any psychoactive effects have subsided. The same does not apply to alcohol, which is metabolised and cleared from the bloodstream within hours.

The Statutory Medical Defence

There is, however, a critical distinction embedded in the legislation. Section 5A of the Road Traffic Act 1988 creates a statutory medical defence for patients prescribed controlled drugs in accordance with a valid prescription ( [Section 5A, Road Traffic Act 1988]). This means that medical cannabis patients who are driving in accordance with their prescription and without impairment are not committing an offence — even if their THC blood levels exceed the 2 microgram limit.

The Crown Prosecution Service guidance on this is explicit. To secure a conviction, prosecutors must prove that the defendant's driving was impaired, regardless of the presence of a specified controlled drug in their system ( [CPS, Drug Driving Offences]). For medical cannabis patients with a valid prescription, the mere presence of THC above the limit is not sufficient for prosecution.

What Counts as Impairment

The concept of impairment is central to the law, yet it remains poorly understood by the public. Impairment is assessed through Field Impairment Tests conducted by trained officers — including the walk-and-turn test, the one-leg-stand test, and the pupil dilation examination. These tests are subjective by nature and have been criticised for their lack of scientific rigour.

The contrast with alcohol is stark. For alcohol, the law sets a bright-line limit: exceed it, and you are automatically guilty of driving with excess alcohol, regardless of whether you actually feel impaired or are driving safely. No medical defence exists for alcohol. For cannabis, impairment must be demonstrated independently of the blood concentration.

This asymmetry reflects the pharmacological reality: alcohol impairs driving ability in a dose-dependent, predictable manner, while the relationship between blood THC concentration and impairment is weak and inconsistent ( [MEDCANN Pharmacy, Driving on Medical Cannabis]). Tolerance, frequency of use, and individual metabolism all affect how THC impacts cognitive and motor function.

Sleep Deprivation: The Forgotten Comparison

A striking frame of reference comes from sleep research. Being awake for 17 hours produces cognitive impairment equivalent to a blood alcohol concentration of 0.05% — the legal driving limit in Scotland and many European countries ( [Williamson & Feyer, 2000, Occupational and Environmental Medicine]). After 24 hours awake, impairment is equivalent to 0.10% BAC — well over the legal limit.

This comparison matters because it underscores the inconsistency in our approach to impairment. We routinely permit — even encourage — behaviour that produces driving impairment equivalent to or greater than moderate alcohol consumption. Yet we fixate on a specific substance when the underlying concern should be functional impairment, regardless of cause.

The broader lesson is that numerical limits, while administratively convenient, are a crude tool for addressing the complex reality of impairment. For alcohol, the relationship between blood concentration and impairment is robust enough that bright-line limits are defensible. For cannabis, the science does not support such a simple approach — and the law, through the statutory medical defence and the requirement to prove impairment, implicitly acknowledges this.

Read more: Physical Effects of Alcohol vs Cannabis →