One cannot simply test employees on drugs and alcohol use, not even with the consent of the employee in question.
Executing alcohol and drugs tests amongst staff is in fact not allowed unless stipulated by law, such as in aviation, railways and shipping. Moreover, in such event alcohol and drugs tests are principally subject to certain conditions. In lack of a statutory regulation allowing alcohol and drugs tests such tests involve a breach of privacy of employees.
Article 9 (1) of the European ‘General Data Protection Regulation’ actually prohibits the processing of ‘special personal data’, as also covers data on health. For more information on the subject please be referred to the informative magazine of the Dutch Personal Data Authority, to be consulted here.