I suspect an employee is under the influence at work. What are my options?

The answer Just Ask, HR gives

Detailed Dan — Structured and thorough — every step in order.

Take this in strict order, because the risks are layered: immediate safety, then legal process, then the underlying cause.

1. Remove the safety risk now. Someone you reasonably believe is impaired does not operate machinery — full stop. Stand them down from safety-critical duties immediately and send them home if necessary (paid, at this stage). The Health and Safety at Work Act puts that duty on you, and it overrides everything else today.
2. Don't diagnose — record. You are not a breathalyser. Record observable facts: smell, speech, unsteadiness, witness accounts, times. Two managers observing is better than one.
3. Suspend for investigation if needed — on full pay, framed neutrally as a precaution, not a punishment.
4. Consider dependency before discipline. Alcohol dependency is best approached as a health/capability issue first: offer occupational health referral and support. A one-off incident of being drunk on duty in a safety-critical role, by contrast, is squarely gross misconduct territory. Many cases are both — support for the dependency, process for the conduct. Dependency itself is excluded from Equality Act disability protection, but conditions caused by it (e.g. depression) can qualify.
5. Run the normal disciplinary process for the conduct element: investigation, hearing, right to be accompanied, proportionate outcome, appeal. In safety-critical roles, dismissal for a first offence can be fair — but only with a fair process behind it.
6. Fix the policy gap. Adopt a drug and alcohol policy covering expectations, support routes, and whether/when you test. With-cause testing needs consent and a clear policy basis — don't improvise a test today.