One way to measure the dangers of various drugs is to examine how toxic the drug is at various levels. Can too much kill you? And how much is too much? Here's an interesting article on what we know scientifically about the matter. Money quote:

The most toxic recreational drugs, such as GHB (gamma-hydroxybutyrate) and heroin, have a lethal dose less than 10 times their typical effective dose. The largest cluster of substances has a lethal dose that is 10 to 20 times the effective dose: These include cocaine, MDMA (methylenedioxymethamphetamine, often called "ecstasy") and alcohol. A less toxic group of substances, requiring 20 to 80 times the effective dose to cause death, include Rohypnol (flunitrazepam or "roofies") and mescaline (peyote cactus). The least physiologically toxic substances, those requiring 100 to 1,000 times the effective dose to cause death, include psilocybin mushrooms and marijuana, when ingested. I've found no published cases in the English language that document deaths from smoked marijuana, so the actual lethal dose is a mystery. My surmise is that smoking marijuana is more risky than eating it but still safer than getting drunk.

Alcohol thus ranks at the dangerous end of the toxicity spectrum. So despite the fact that about 75 percent of all adults in the United States enjoy an occasional drink, it must be remembered that alcohol is quite toxic. Indeed, if alcohol were a newly formulated beverage, its high toxicity and addiction potential would surely prevent it from being marketed as a food or drug. This conclusion runs counter to the common view that one's own use of alcohol is harmless.

The least toxic drug known to humans is now illegal. The most toxic is available at Safeway. None of this makes any sense at all. And yet we continue to imprison people for ingesting substances far less harmful than others freely available. One has to wonder what the prohibitionists are smoking. Maybe nutmeg.