Problems like drug and alcohol abuse aren't easy to deal with. Simply setting down rules and then disciplining employees who break them is rarely the best approach. Instead, you should look for ways to support employees who have a problem.
You may be able to provide a sympathetic ear yourself but, as these are complicated matters, specialist advice may be needed. You should encourage employees to make use of support services. For example, an employee's GP may be able to provide help and advice or refer them to a specialist.
Occupational health and welfare
Some businesses also provide an occupational health service for their employees. As well as responding to any problems that arise, this can help to promote employee welfare and prevent problems arising in the first place.
You'll need to investigate whether the costs would be matched by reductions in illness and absence.
Find your local Trust on this list of Health and Social Services Trusts.