do workplaces have a problem with ‘drug thinking’?

September is Recovery Month, but approaches to employee substance use have barely evolved. Dr Georges Petitjean explores how businesses can keep pace with changing medicine, legislation and the modern workforce.

A laboratory can answer the wrong question perfectly.

An employee provides a sample. The result is positive for cannabis. The analysis may be technically sound, the chain of custody secure and the paperwork satisfyingly complete. But what, exactly, has the test shown?

That has become harder to answer since 2018, when certain cannabis-based products for medicinal use were moved into Schedule 2 of the Misuse of Drugs Regulations, creating a lawful route to prescription.

Recreational cannabis did not become legal. Yet cannabis can now sit, depending on context, on both sides of a workplace ‘drug’ policy: prescribed medicine and illicit drug.

The Care Quality Commission reported a 130% rise in private prescription items for unlicensed cannabis-based medicinal products between 2022-23 and 2023-24.

Those are prescription items, not patients, but the rise makes medical cannabis increasingly difficult for employers to regard as marginal.

The problem with a positive test

The problem is that workplace ‘drug’ thinking can ask one result to carry several meanings.

A positive cannabis test can indicate exposure. Depending on the test, it may tell us something about how recent that exposure was.

Urine can remain positive long after acute effects have passed; oral fluid is generally more closely related to recent use. Neither result, by itself, establishes that somebody is impaired at work.

A prescription does not make the interpretation simple either. An employee may be using cannabis as prescribed, may also be using illicit cannabis, or both.

A routine positive result does not establish the source of every detected cannabinoid, any more than it establishes current impairment.

Lawful prescribing changes the medical and legal context; it does not turn a toxicology result into a complete account of behaviour or fitness.

Impairment

None of this means cannabis is harmless. Tetrahydrocannabinol (THC), the main compound in cannabis that is responsible for the ‘high’, can impair attention, tracking and other abilities relevant to, for instance, driving and safety.

Nor is there a simple biological number that reliably converts a THC concentration into an individual verdict of ‘fit’ or ‘impaired’.

Research has found blood and oral-fluid THC concentrations to be relatively poor indicators of cannabis-induced impairment.

Employers are therefore left with a practical difficulty: presence and impairment are different, but impairment is not always easy to measure directly.

In many non-safety-critical roles, that may support greater emphasis on observable performance, conduct and occupational-health assessment. In a safety-critical role, however, the consequences of getting the judgment wrong can be catastrophic.

HSE guidance recognises a case for drug screening in roles involving safety-critical decisions.

A lawful prescription therefore cannot be treated as an automatic certificate of fitness.

The better approach is to start with the risk: what are we trying to manage, and what does this particular evidence actually tell us?

Justifying tests

The Information Commissioner’s Office advises employers to justify testing by reference to safety, fitness or another legitimate purpose, to use proportionate methods and avoid testing merely to discover illegal drug use in someone’s private life.

It also warns that some tests may fail to distinguish neatly between illegal drug use and legitimate medication.

A sensible policy should give employees a confidential route to disclose prescribed medication where it may affect safety or testing, without unnecessary medical detail circulating through line management.

What matters is whether the medication, the underlying condition or both affect fitness for the role. Occupational health can help separate the clinical issue from the disciplinary one.

A recent rail case shows how quickly these categories can collide. In Truman v SPL Powerlines UK and others, a worker prescribed medical cannabis for a disability failed a test connected with a safety-critical role.

In May 2026, the Employment Appeal Tribunal allowed part of his disability-discrimination appeal against Network Rail and sent it back for reconsideration.

It did not decide that prescribed cannabis users must be accepted for safety-critical work. Prescription, a positive test and fitness remained different issues, according to the court.

Another distinction

Cannabis use is not synonymous with cannabis addiction. Prescribed use is not evidence of addiction; illicit use is not automatically evidence of addiction either.

An employee struggling with addiction should not become merely a disciplinary category because the relevant substance appears under “drugs and alcohol”.

That is where culture matters alongside policy. Organisations cannot remove every risk, but they can make it easier for people to seek confidential advice, treatment or support earlier.

That does not require employees to ‘disclose’ everything to their employer: some may seek help entirely outside work.

What employers can influence is whether their culture makes asking for help safer or harder, and whether managers understand that support and safety are not opposites.

Employers still need rules. They may need testing, temporary removal from safety-critical duties, occupational-health advice, adjustments, investigation or disciplinary action. None of that requires pretending those decisions are interchangeable.

The laboratory has done its job when it tells us what it has detected. The harder task belongs to the organisation: deciding what that finding means for this employee, in this role, at this time.