On the topic of illegal substances, the one that I find to be the most interesting is LSD. It was discovered to be hallucinogenic by Albert Hofmann in 1938, and quite a lot has happened since then.
Although I have never tried it myself, I have some friends who have. One friend agreed to answer some questions about the experience for this post, but has asked to remain anonymous for obvious reasons. He said that first of all, LSD is not powerfully hallucinogenic for him and that the visual effects are subtle unless he goes into higher doses (though this may not be the case for everyone; it is a mysterious and subjective experience). He said that it mostly seems to intensify his emotions and expand his consciousness. He says that any emotions that he feels while tripping seem to be hugely magnified. If he is sad, he feels unbearably sad and uncomfortable, and if he is happy, he feels euphoric. When asked what he meant when he says that his consciousness was expanded, he said it was hard to explain but tried his best anyways. He said that it felt as though his mind had burst out of a box that had been inhibiting his thoughts prior to using the LSD. He said that it was as though his mind had reached a heightened state of existence. This sounds like a very enlightening and unique experience to me.
LSD also has many therapeutic effects on your mind. Many people report that LSD helps them introspect very thoroughly and from an objective viewpoint, allowing them to make realizations about themselves that can be life-changing. This includes ridding people of alcoholism and addiction to other drugs. Some studies professed a 50% success rate at curing alcoholism, whereas Alcoholics Anonymous only has a 10% success rate (http://en.wikipedia.org/wiki/Lysergic_acid_diethylamide#Alcoholism).
Such an amazing drug must have its downsides to your health, right? After a quick search on Google, I discovered that there have been no deaths caused by LSD, and that there is no evidence for long-lasting effects on the brain or other parts of the body. It is possible for LSD to make you become schizophrenic, but only if you have a history of schizophrenia in your family. There is also a risk of developing HPPD, which causes some hallucinogenic effects to last permanently after your trip (http://en.wikipedia.org/wiki/Lysergic_acid_diethylamide).
I am not endorsing the illegal use of LSD, but I am curious as to why it is illegal. I personally believe that all psychoactive chemicals should be legalized, and I will post about why that is tomorrow.