Impairment vs Prescription: Why a Positive THC Test Is Not the Same as Being Impaired

Nathan Williams, a patient prescribed medical cannabis since 2020 for chronic pain, was stopped twice by police in 2023 and refused a roadside saliva test both times, citing his prescription. 

He was charged with failing to provide a specimen under the Road Traffic Act and cleared in March 2024, when magistrates ruled his prescription gave him a ‘reasonable excuse’ to refuse the test. 

Roadside disputes and workplace parallels

While, on the face of it, these appear to be distinct situations, the parallels should not be dismissed. A drug test detects the presence of THC, which can remain identifiable for days or weeks after use, telling an employer nothing about whether that person is impaired at the moment they clock in.

‘Releaf Protect cases demonstrate how quickly those assumptions can displace any assessment of the employee’s actual functioning. A lab test or the name of the medicine cannot answer that question alone.’ – Rupa Shah

Policing has started to catch up with that distinction faster than employers have. The National Police Chiefs’ Council approved formal guidance for officers in early 2026, instructing forces in England and Wales to treat prescribed patients as legitimate ‘until proven otherwise’, now adopted across 43 forces. 

CQC data put UK prescription volumes at roughly 300,000, having doubled within a year, yet no equivalent, cannabis-specific guidance yet exists for employers.

You can read the first chapter in our ‘Medical Cannabis in the Workplace‘ series here. 

 

Individual assessment over generalisation

For safety-sensitive roles, that scrutiny should not be relaxed, but applied to the individual rather than the category.

‘Employers should assess the employee’s actual condition against the specific demands and risks of the role, using occupational health where available or appropriate. 

‘They should first consider the least restrictive effective control – for example altered dosing times, temporary duties or additional review. Excluding the employee should be the last considered option, not the starting point.

‘We also see policies designed for recreational drugs applied arbitrarily to prescribed medicines.’ – Rupa Shah

These issues are repeatedly raised by patients contacting Releaf Protect, with recurring themes including instant dismissal, treating a positive THC test as proof of impairment, and demanding complete medical records where such detail is not necessary. 

Where HSE guidance falls short

The Health and Safety Executive publishes general guidance for safety-critical tasks, covering driving, machinery operation and work at height, but this stops short of any specific guidance relating to medical cannabis, leaving employers to translate broad principles into individual decisions largely on their own.

‘HSE does provide some guidance for safety-critical tasks, but there is little medical-cannabis-specific guidance. Our discussions with employers show that they need help translating those general principles into individual decisions.’ – Rupa Shah