Many people have conditions that require prescribed drugs to help them live a normal life and could not work without them. Taking prescribed drugs at work is not illegal.
But it is a criminal offence under the Misuse of Drugs Act 1971 for any person knowingly to permit the production, supply, possession and use of controlled drugs on their premises except in specified circumstances such as where they have been prescribed by a doctor.
Employers also have general duties to protect workers' health and safety under the Health and Safety at Work Act 1974 (HASAWA), and to carry out risk assessments under the Management of Health and Safety at Work Regulations 1999. This would include the use or presence of drugs and alcohol at work if there appears to be a risk to workers.
Under section 7 of HASAWA, employees have a duty to take reasonable care of their own health and safety and that of others who might be affected by their acts or omissions at work.