When you're a child, they tell you not to talk to strangers. But adults have to talk to strangers, from the prospective employer who interviews you to the plumber you hire to fix your sink. Part of growing up is learning how to tell the difference between good people and bad people so that you don't need an "all strangers are bad" rule.
The same is true of drugs. An adult considers what a drug will do to their body, and decides whether or not to take it. An adult also considers things that a child wouldn't, such as the fact that drugs bought on the street probably aren't whatever they claim to be. You don't need a simple "always say no" rule when you can make an informed decision.
You shouldn't always say yes, either, even to a doctor. If you're not comfortable taking a certain medicine, you don't have to take it. Sometimes you will try a medicine, find that the side effects are worse than your original condition, and decide to stop taking it.