Ah drugs. The other can of worms. I'll throw in my two cents I think.
LSD - This is quite a major hallucinogenic. If legal you could limit the potency of it, but what's to stop people taking a whole heap at once? In NZ when we had party pills (BZP based) the recommended dose was something like 2 a night. People would take 20 in one go.
Also because LSD is a hallucinogenic, people under the influence can be quite unpredictable. You can't see what they're currently tripping balls to. LSD experiences can range from mild (no hallucinations but impairment of judgment, sense of direction, memory and decision making) to extreme (hallucinations of all five senses, extreme impairment of judgment and decision making). Nothing quite like dealing with someone who things everyone are devils out to get them.
Marijuana - No active addictive ingredients, however THC can trigger mental health problems such as Schizophrenia in a small number of people through either a predisposed genetic conidition or even regular use.
However given that the effects when under the influence of marijuana are no more profound than alcohol, then with reasonable controls in place marijuana should be at the very least decriminalized. (When I say controls I mean that you shouldn't just be able to walk down the street smoking a joint.)
As a historical fact there is some evidence that marijuana was an authorized alternative to alcohol because it mean that the soldiers could get off their face but not have a hang over the next day.
However with marijuana you still have to think about the effects of smoking it and the damage it can cause through ingesting smoking into the lungs (throat cancer, lung cancer, even rotting teeth.) Things like vaporizers a recommended because then you are mostly inhaling the THC not all the other crap as well.
Cocaine, heroin, methamphetamines, and other derivatives - These are highly dangerous due to the addictive and sometimes personality changing properties of these substances. While I agree that people should have the choice to take what they want, it's hard to defend that view point when what they are taking can dramatically change someone's behavior.
Some of these things I can speak from personal experience, others I have seen first hand happen to friends and I'm also interested into the research of all these drugs. So um. Go drugs?