Health and Safety at Work etc Act 1974, section 2
Management of Health and Safety at Work Regulations 1999
It is an offence under the Misuse of Drugs Act 1971 for any person knowingly to permit the production, supply or use of controlled substances on their premises except in specified circumstances (e.g. when they have been prescribed by a doctor).
You should also be aware of duties under the Road Traffic Act 1988 and the Transport and Works Act 1992. Drivers of road vehicles must not be under the influence of drugs while driving, attempting to drive or when they are in charge of a vehicle. Certain rail, tram and other guided transport system workers must not be unfit through drugs while working on the system. The operator of such a system must exercise all due diligence to avoid those workers being unfit.
Drug and other substance (e.g. solvent) misuse is everyone's concern. In the context of work, not only does it damage the misuser's health, but it can cost employers through absenteeism and reduced productivity. It may also increase the risk of accidents. Employers should adopt a substance misuse policy, in consultation with their staff. This policy should aim to support affected employees rather than punish them, though your policy must say that possession or dealing in drugs at work will be reported immediately to the Police. If an employee admits to being a drug user, your policy should seek to help them rather than lead simply to dismissing them.
Some employers have decided to adopt drug screening as part of their drug policy. If you think you want to do the same, think very carefully about what you want screening to do, and what you will do with the information it generates. It is also important to consider the drug testing process itself including the type of testing, how the sample is collected and the security of the sample from contamination. Advice has been prepared by the European Workplace Drug Testing Society (EWDTS) to ensure the drug testing process is reliable and accurate.
Drug screening by itself will never be the complete answer to problems caused by drug misuse.