In the UK, prescribing cannabis-based products for medicinal use is restricted to only those clinicians listed on the Specialist Register of the General Medical Council.
UK law requires that drivers tell the Driver and Vehicle Licensing Agency (DVLA) about any medical condition that could potentially affect their driving. For certain conditions that are deemed to pose an inherent threat to safe driving, including epilepsy (for which medical cannabis can be prescribed), DVLA must be informed. DVLA produces guidance for medical professionals on assessing fitness to drive to help them advise patients on whether to notify the DVLA of a medical condition. The guidelines give an outline of the elements required for safe driving (such as attention and concentration, good reaction time, and coordination) but apart from conditions where there are clearly laid down criteria for driving eligibility based on clinical history, such as epilepsy, it is down to the individual driver to judge whether their ability to drive safely is impaired.
According to the NATCen Social Research, the impact on road safety of increased numbers of prescriptions for medical cannabis is not clear. Medical cannabis may make people safer drivers by reducing impairments from illnesses which medical cannabis is treating, including pain, stiffness, and mental distraction due to anxiety and ADHD. On the other hand, the psychoactive effects of THC clearly increase impairment through drowsiness and other cognitive impacts. The direct impact of such impairment on road safety is not clear, as patients will receive medical advice to only drive if they do not feel impaired. In some cases, people may become willing to drive if there is an improvement in their symptoms but still be somewhat impaired; some patients also report that their driving improves when using medical cannabis, which implies that they were driving previously. This suggests that in practice there are different degrees of impairment among drivers, even though legal rules and medical guidance are that patients should not drive if their driving “is impaired”.
In line with DVLA guidance, prescribers should give patients advice on driving while using medical cannabis, as they would with any other medication. This advice covers the main point that the patient should not drive if they feel impaired, and sometimes includes more specific guidance to support safe consumption. Prescribers should also consider the interaction of medical cannabis with other medications and monitor patients’ responses to their medication, including those likely to impair driving.
Different medical cannabis products contain different levels of THC and CBD and different THC:CBD ratios, which can impact on impairment in different ways. Studies offer a mixed picture of the interaction of CBD and THC in relation to driving impairment.
There has been some concerns regarding if limits or impairment form the best basis for offences of driving under the influence of cannabis due to the situations such as long-term cannabis users may have elevated levels of THC in their blood despite not having used cannabis recently and would therefore not be impaired to drive safely. Long-term users might be criminalised without presenting an elevated risk, and equally that other patients on lower doses and with less THC in their blood may have much greater impairments, depending on other contextual factors: “Some patients on higher doses of medical cannabis may reach that [upper limit] but might not be affected; while some patients on lower doses, who are nowhere close to the legal cut-off are severely affected and should not drive.”
Although the evidence suggests that per se limits may not be an accurate interpretation of impairment in all cases, the UK drug driving legislation must regulate liability. In practice, enforcement of drug-driving laws based on impairment is available to police and prosecutors. The
per se limits aim to deter drug-driving regardless of impairment, and the medical defence provides prescribed drug users do not risk being prosecuted for drug driving simply for using their medication as advised.