An adult should be allowed to do whatever s/he likes as long as it is not harming others. If harm to others is done - even under the influence - then punishment is due. But as long as somebody just does something in private - it must be okay.
I have plenty of experience with psychedlics - be it LSD, Psilocibin, DMT or 4-ACO-DMT. I know from own experience that a bad trip can be very unsettling. But as an adult it is first of all my right to take that risk and second of all it is a maturing experience.
The state should not be allowed to prohibit anything when done in private. The concepts behind drug prohibition are pretty much the same as behind prohibition of homosexual practices. People also say this should not be allowed because it is bad and young children will be molested etc pp.