I don’t think being a mental patient and being a slave are the same thing. Slaves actually did physical labour for their masters, whereas psychiatric patients consume a substance for which they or the government pay. Those are two very different activities. The psychiatric patient is not compelled to spend all their waking hours performing unpaid labour for the doctor. In fact, the drugs can make it impossible to do any sort of work if they make you enough of a zombie. I think we need a different sort of metaphor here. Perhaps, if you are forced to take psychiatric drugs, you are a prisoner (e.g. take a look at “Tranquil Prisons” by Erick Fabris). But slave, I don’t think so.