Replace the words "drug" and "medicine" by "chemical" or "bioactive substance". You see, that's what we must talk about if we talk about legalizing "drugs". One person's drug or medicine is another's poison (Datura is a good example, interesting for discussion because it's legal to own as a plant). If you're fine with criminalizing the sale of contaminated meat, then you are implicitly relying on the government to protect against certain bioactive substances. You're clearly willing to make the security/liberty sacrifice, as are most reasonable people.

EDIT: You'd better serve your interests by advocating for a well-regulated narcotics market with proper safety controls and the distinct possibility that certain substances will just have to be illegal. That's much sounder than saying "legalize drugs because I prefer liberty over safety, except sometimes".