What is the ruling on selling medicine to someone suspected of misusing it (as a drug or similar substance)?

It is not permissible to sell medicines to those who use them as narcotics, for this is a form of cooperation in sin and aggression, which is forbidden. If you know or strongly suspect that the buyer will use them as narcotics, it is forbidden to sell them to him. If you know or strongly suspect that he will use them only for permissible purposes, then there is no harm in selling them to him. If both possibilities are equally likely in your estimation, then act according to the predominant practice of users, which is the use of medicines for permissible treatment. It is more prudent not to sell to someone whose intention is doubtful.