This is a very good question and one that is very important. Here are a few points that are important to consider.
First, you need to treat each person as an individual. In other words, each case is different. For some, a stern warning and some reasoning might do the trick. If the person is an addict, then more drastic measures will be needed.
Second, in some cases the person needs to be admitted to a hospital or a drug rehabilitation center to get their bodies off the drugs. The withdrawal process is very difficult. In light of this external help is essential.
Third, a drug user also needs to have good follow up. For this reason, some sort of support group would be a good step. These support groups can create a social environment that allows people to stay off of drugs and thrive.