Will prescribed cannabis show up on a workplace drug test?

A workplace drug test measures what is in your body, not why it is there. It will not distinguish a prescribed medicine from an illicit one, which makes disclosure a practical necessity rather than a matter of principle.

What the test actually detects

Workplace testing usually looks for THC metabolites in urine or saliva. These are breakdown products that linger well beyond the period in which the medicine has any effect on you.

THC is fat-soluble, so it accumulates with regular use and clears slowly. After sustained daily use, urine testing can remain positive for several weeks after the last dose. Saliva testing has a shorter window, closer to the period of recent use.

The practical consequence is that a positive result tells your employer very little about whether you were impaired at work — but a positive result is what the policy responds to, not the nuance. Detection windows vary a great deal between individuals, and no one can promise you a specific number of days.

Whether to tell your employer

There is no general legal duty to volunteer your medical information to an employer. There are several situations in which not doing so is a poor idea anyway.

• Your contract or the drug and alcohol policy may require declaration of prescribed medicines that could affect performance — read it before deciding
• In a safety-critical role — driving, machinery, heights, healthcare, rail, aviation, offshore — there is usually a positive duty to declare anything that could impair you
• Where your employer has a health and safety duty toward you and others, withholding relevant information can undermine your own position later
• If you may need adjustments, disclosure is what triggers the conversation

Where you do disclose, do it in writing, to occupational health rather than to a line manager where that option exists, and before any test rather than after one. Occupational health is bound by medical confidentiality and normally reports only fitness for work, not your diagnosis.

The Equality Act angle

The medicine is not protected. The condition may be. If your underlying condition meets the Equality Act definition of a disability, your employer has a duty to make reasonable adjustments and must not discriminate because of it.

A condition qualifies where it is a physical or mental impairment with a substantial and long-term adverse effect on normal day-to-day activities. Many of the conditions treated in this field meet that description.

Reasonable adjustments might include altered hours, a change of duties, or a different approach to a blanket testing policy. What is reasonable depends on the role and the employer, and a safety-critical duty can legitimately outweigh an adjustment that would otherwise be reasonable.

If you are facing disciplinary action over a positive test while holding a valid prescription, get advice early — from ACAS, a trade union, or an employment solicitor.

Roles where the answer may simply be no

Some sectors operate under statutory or regulator-imposed regimes where a prescription does not create an exception. It is better to know this before starting treatment than after.

Aviation, rail safety-critical work, maritime roles, some armed forces positions, certain healthcare roles and jobs requiring particular vocational driving entitlements can all carry rules that a prescription does not displace. Some employers with zero-tolerance policies apply them regardless of medical status, and whether that is lawful in a given case is fact-specific.

Raise this with your prescriber during the consultation. A specialist who knows you drive an HGV or work on the railway should be factoring that into the decision, and may well advise against treatment.

One caveat before you rely on any of this

This is general information about how the rules on employment and drug testing work, not legal advice, and it cannot take account of your circumstances.

The law changes, enforcement practice varies, and the details of your own case matter. If something here could affect your licence, your job or your liberty, get advice from someone qualified to give it — a solicitor, your prescriber, or the relevant authority itself. Where we can point you at the primary source, we have.

Patient guide · Written by the Flora House editorial team · Last reviewed: 8 September 2026