Work and private life
In principle, work and free time are separate. Your employer can therefore hardly sanction you for something that is purely private and takes place in your free time.
Nevertheless, reality is more complicated. There are quite a few exceptions to this principle. Suppose, for example, that you show up at work drunk or under the influence and are unable to do your job. Or worse, you are a danger to yourself or others. Then it is logical that your manager will speak to you and possibly ask you to go home.
In certain jobs, any form of drink, drugs or pills is out of the question. Often for safety reasons. Think about bus or train drivers, pilots, police officers. The labour regulations are an important document in this respect. If a job does not allow any use, or if your employer wants to subject you to a breath or drug test, then that regulation is included.
Drink, drugs, pills and your performance
Alcohol, medicines and drugs can become a problem for your employer if they make you perform less well at your job. Reduced performance, poorer performance or deteriorating working relationships are a reason to call someone to account for his working behaviour and the role that alcohol, drugs or pills play in this. Poor performance can therefore be a reason to dismiss you.
How this works out in practice depends largely on the alcohol and drug policy of your employer. The main purpose of such a policy is to avoid performance problems and, in case of problems, to look for a solution together. But a policy also includes sanctions. For example, for employees who are not prepared to tackle their problems. Or when you make a serious mistake due to your alcohol or other drug use.