Every drug has its physical costs. Some more than others. Alcohol, for instance, has no non-harmful dose-- even one glass of wine with dinner is bad for your body-- but many people drink throughout their long healthy lives.

That said, "harmful" drug use in this case likely refers to exceptional harm, perhaps including addiction or overdose.

You can be given, essentially, heroin at the hospital if in extreme pain. Would you consider this to be "non-harmful"? I would.