It often is, especially in safety-sensitive jobs or if you drive.
The Transport and Works Act 1992 makes it an offence for certain workers to be unfit through drink or drugs while working on railways, tramways and other guided transport systems.
The Road Traffic Act 1988 makes it an offence for anyone unfit through drink or drugs to drive.
There are lots of other examples.
Employers also have general duties to protect workers' health and safety under the Health and Safety at Work Act 1974 (HASAWA), and to conduct risk assessments under the Management of Health and Safety at Work Regulations 1999.
There is evidence of a growth in the use of alcohol and drug testing at work. You can find more information about this on our Drugs and Alcohol page as well as the TUC’s page with resources about Drugs and Alcohol testing.