Employers have to have consent if they want to test for drugs. Usually this is when they have a full contractual health and safety policy, which should be in the contract or staff handbook.
limit testing to employees that need to be tested
ensure the tests are random
not single out particular employees for testing unless this is justified by the nature of their jobs
Workers can’t be made to take a drugs test but if they refuse when the employer has good grounds for testing, they may face disciplinary action.