The Animal World Has Its Junkies, Too
Phoghat writes "Research scientists have used many animal species in investigating mind-altering drugs, but it may come as a surprise to learn that animals in the wild — from starlings to reindeer — also make use of psychoactive substances of their own accord. It seems that many of these species have a natural desire to experience altered states of consciousness, and man may well have found his way to some of his favourite recreational drugs by observing the behaviour of animals."
I can only imagine the teenager replying to the judge: "But your honor here a picture of a Reindeer seeking the hallucinogenic fly agaric mushroom".
Funny you should choose the fly agaric mushroom for your example. Fly agaric is only a controlled substance (illegal to possess) in one U.S. state: Louisiana (source). Elsewhere, you can munch on them as much as you want (note: most people find the effects very unpleasant).
It is perhaps worth noting that although possessing/eating fly agarics is not illegal, it is a violation of FDA regulations to sell them for food or drug purposes.