Why?

No reason required. People can already buy incredibly dangerous things with a doctors permission, as if a doctor actually knows what other compounds a person is consuming to begin with. Doctors are not omniscient and patients lie. Most of them barely even know the compounds that are FDA approved to begin with. Be honest, most of them barely remember 10% of what they were taught in medical school and the schools even state that half of what they will be taught will not be relevant or will be entirely wrong by the time they graduate.

Again, I can buy apples and apricots without permission. There is nothing capable of more risk or harm that has ever been approved by the FDA than apples and apricots.