Why can’t Congress pass a law making prescription drugs (the non controlled ones that are habit forming) something you can buy OTC like Tylenol and Advil?

What would be the dangers in doing that? At least the black market for prescription drugs would go away.

Why can’t Congress pass a law making prescription drugs (the non controlled ones that are habit forming) something you can buy OTC like Tylenol and Advil?

What would be the dangers in doing that? At least the black market for prescription drugs would go away.

I have to do a research project and I have to find a lethal dose
for my body. Drugs such has Tynenol, advil, pepto bismol,
and such.

How do you calculate that?
Also, how do you find the ld50 of the drugs?

Pleaseeee helppp.