There are certainly situations in which these substances are very useful, and I think we have to be careful not to let our preconceptions stop us from using substances to alleviate suffering where appropriate. It’s a complex issue, with some debate as to when it’s appropriate to prescribe these things and when other avenues would be more suitable. I think it’s mostly a matter for doctors.
That said, I think such substances do highlight interesting questions about how we perceive ourselves. I’m on a medication which, as a side effect, calms me down. It’s changed how I behave, at least a little bit, but what does it mean if my altered chemical make-up has changed my behaviour? To me, it makes me wonder how much my behaviour has always been shaped by chemicals, natural and otherwise, and what that means for my own free will. I’m not sure I have any answers.