An employer may ask you for a drug test certificate when recruiting for work that demands precisely specified abilities, but may only do so if you have been provisionally selected for the position. The employer may then turn down your application if you fail to provide the certificate or if the test result is positive.
An employer may ask for a drug test certificate during your employment in certain duties specified by law if there are justified grounds for suspecting that you are under the influence of drugs at work, or that you are addicted to drugs. A drug test may only be conducted by a health care professional.
Workplaces where tests are done must also have a substance abuse programme, and this must also include preventative measures.