Generally, the safety of a given substance can be determined through its therapeutic index. The therapeutic index for a substance is calculated by taking the amount of the substance that will induce toxicity in 50% of a population divided the dose that will induce the effect of the drug in 50% of the population. In short, the ratio of how much will be (acutely) toxic to how much will make you high.
Here are some therapeutic indexes for common drugs [0]:
Heroin - 6
Alcohol - 10
Cocaine - 15
MDMA - 16
LSD - ~1000
THC - > 1000
Alcohol has very dangerous acute effects and it's very possible to overdose on alcohol. Compare to THC where a toxic dose is on the level of 15g. Assuming that THC is approximately 5% of the content of the cannabis [1] this would mean smoking 300 grams of cannabis (with 100% absorption by the body). That would make for ~600 joints (with a joint having about half a gram of cannabis) to get to a level of toxicity.
So relatively speaking, Alcohol (at least in the short term) is quite dangerous.
Those are great statistics for judging the likelihood of overdosing (although, I'm not sure it's the only factor to consider - the physically small amount of cocaine needed to OD probably mitigates its higher therapeutic index).
If you want to evaluate danger there are other factors that need to be considered -- i.e. people "safely" high on MDMA may dehydrate, and people "safely" high on LSD may jump off of a bridge.
people "safely" high on LSD may jump off of a bridge.
Sober people may jump off a bridge too. Should we outlaw sobriety? The "man on LSD thinks he can fly and jumps out of window" myth is largely that: a myth. If I recall, it was inspired by a real story of LSD experimentation on some kind of federal agent, who was given a good dose of LSD without his knowledge, thought he was going mad, and threw himself out of a window to make it end.
Long story short, a student took LSD on a class trip, which involved a visit to the Capilano Suspension Bridge. The student didn't jump off the bridge, but climbed a railing on an observation deck and fell.
I am completely against prohibition of any drug, however, I can speak from experience that LSD can genuinely drive you temporarily insane, even at relatively small doses.
More than once I have been convinced (even though I knew I had put a little piece of paper on my tongue) that what was happening to me was outside of the context of 'tripping.'
One time I would have committed suicide had I been convinced that it would have worked. I was completely under the impression that it would only be painful and not really do anything but sort of hit a 'reset' button into what was at the time essentially hell.
I'm categorizing distilled alcohol (i.e., liquor) differently than beer and wine, as I think the former is significantly more dangerous. I could be wrong, though - I wonder where it would end up on that scale if the different forms were rated distinctly.
Here are some therapeutic indexes for common drugs [0]:
Heroin - 6
Alcohol - 10
Cocaine - 15
MDMA - 16
LSD - ~1000
THC - > 1000
Alcohol has very dangerous acute effects and it's very possible to overdose on alcohol. Compare to THC where a toxic dose is on the level of 15g. Assuming that THC is approximately 5% of the content of the cannabis [1] this would mean smoking 300 grams of cannabis (with 100% absorption by the body). That would make for ~600 joints (with a joint having about half a gram of cannabis) to get to a level of toxicity.
So relatively speaking, Alcohol (at least in the short term) is quite dangerous.
[0] PDF - http://web.cgu.edu/faculty/gabler/toxicity%20Addiction%20off...
[1] http://en.wikipedia.org/wiki/Cannabis_%28drug%29#Potency