The misuse of drugs and alcohol can be a serious workplace issue. Not only can their use lead to significant health problems but anyone under the influence of drugs or alcohol can be a hazard to themselves and others. You as an employer have a duty to ensure your employees' health, safety and welfare at work. This means as the employer you would be committing an offence to knowingly allow an employee to work whilst under the influence of alcohol or drugs where it poses a threat to the health and safety of that employee or other people. To knowingly allow controlled drugs to be illegally made, supplied or used on your premises is also an offence.
The UK Government highly recommends, and it is best practice, that as an employer you should have a written policy on drugs and alcohol in the workplace and the strictness of the policy and how it is put into practice will vary depending on your industry. A good policy should offer advice for workers who may have a problem, but also clearly explain any possible disciplinary actions. It should encourage workers to seek help and guide workers to such sources. The policy should apply to all workers, although certain elements, e.g. testing of those doing safety critical jobs, may only apply to some workers.