A person has a basic right to harm himself, as long as he does not harm others in the process. That’s because everyone of us is the owner of his mind, body and property, and is solely responsible for their use and abuse.
All arguments that do not favour that, like that a person thus contributes less to society, and becomes a burden on it are pretty silly. That’s because a person is never obliged to contribute to society (“First, do not harm”), and the society at large is not obliged to support its members. (It would be Socialistic to claim otherwise.)
Thus, a person must be allowed by law to consume drugs. I’m not saying it is desirable if a person exercises this right. I’m just saying that a person should be allowed to do so, just as he is allowed to smoke, drink alcoholic beverages, deteriorate in personal hygiene, give away money for no good reason, act stupidly, etc. These are all harmful activities, yet perfectly legitimate.