Considering all the people whose lives were ruined by prescription medicine, I think his worries are justified. There are very real downsides to taking meds.

The "bad thing", for example, could be developing bad side effects to an anti depressant he's been taking for years, but now he's chemically addicted and when he tries to quit, he goes through extremely weird-feeling withdrawals. So either he deals with the side effects of continuing, or with the constant nagging pain/discomfort/emptiness of quitting.

Either way, his life changed for the worse, maybe permanently.