Yesterday, we talked a lot about "controlled substances". The professor who oversees my research (well, who designs it and lets my coworker and me run with it) is a biochemist; she's taken lots of classes in neuropharm, neuropsych, etc. (She was also an aspiring psychology PhD.) Consequently, my coworker and I usually ask her any bio-related questions we might have.
We mostly talked about the damage different drugs do to your brain. According to her, the least damaging "controlled substance" for your brain is marijuana; it's even less dangerous than alcohol.
Now, marijuana does some bad stuff. If you chain-smoke it, it's worse for your lungs than cigarettes. Marijuana has more tar and is rarely filtered; the one exception we noted was in the use of a water bong. Also, apparently, the more THC, the less tar. However, the experts are still out on whether or not smoking pot does sustained brain damage in measurable levels (ie more brain damage a year after quitting than normal living does). It certainly does impact brain activity in the short term. For more information, you can always look up some research on the subject.
I also learned that meth is the worst drug, overall, because it does concentrated brain damage. Many drugs damage the whole brain: this is just like everday living at a more accelerated pace.
Alcohol hurts your brain in a fairly simple manner: osmosis. It's a molecule that floats around in your blood stream. In the process of osmosis, water moves from areas of high concentration to low concentration. As the alcohol level of the blood increases, water leaves the body (including the brain) to dilute the alcohol in your blood, thereby causing fuzzy headedness ("buzzed"). Apparently, the same thing happens with salt. You can gradually build up a tolerance to the substance that makes it life threatening to go cold turkey. (warning: possible urban legend -- if you can back this up with an article or reputable source of some sort, please let me know) My professor told us about a kid who was literally eating tablespoons of salt. They took him to the doctor, who immediately prevented him from taking in more salt. In a few hours he died, because his body had built up a tolerance for high salt levels. Your body does the same thing for alcohol and many other substances; hence, we have detox for alcoholics instead of throwing them in jail to go cold turkey and die.
Rapid consumption also has it's dangers; another example of the benefits of tolerance AND the dangers of rapid consumption can be found in an observation of drinking games. Another fun, yet unsubstantiated, fact: pounding a fifth of alcohol in one shot, or pulling a John Belushi, is lethal for almost everyone. It's too much all at once.
**REVISION** Check out Alice! for more info on the effect of marijuana. This goes to show that, even if marijuana is less harmful than other stuff, it's still harmful. The best way to prevent harm is to take no drugs at all. Think about it.