>Here's my .02
This is a chart of LD50s. The "LD50" is the dose of a substance that will kill a selected species of experimental animal 50% of the time.
All LD50's are normalized to milligrams/Kilogram of animal's body weight.
"Typical Human Dose" info is taken from various sources, including "Burger's Medicinal Chemistry", "Journal of Psychedelic Drugs", "The Merck Manual", "NIOSH Registry of Toxic Effects of Chemical Substances", cigarette packages, No-Doz bottles, etc.
The NIOSH Registry of Toxic Effects of Chemical Substances also lists most published LD50 measurements, but does so uncritically. It's quite useful as a point of reference.

Just compare Psilo, MDMA, THC, and mescaline to that of Caffeine.
Also, for instance, 46 mg/kg of LSD seems very very fatal compared to the other substances, but that's the equivalent of taking 34,500 hits if you are a 165 pound male. (that is based on 100 mic hits)
This is also one of the most common myths that's associated with Marijuana and driving. This is taken from Erowid;
6. Legal marijuana would cause carnage on the highways
Although marijuana, when used to intoxication, does impair
performance in a manner similar to alcohol, actual studies of the
effect of marijuana on the automobile accident rate suggest that it
poses LESS of a hazard than alcohol. When a random sample of fatal
accident victims was studied, it was initially found that marijuana
was associated with RELATIVELY as many accidents as alcohol. In
other words, the number of accident victims intoxicated on
marijuana relative to the number of marijuana users in society gave
a ratio similar to that for accident victims intoxicated on alcohol
relative to the total number of alcohol users. However, a closer
examination of the victims revealed that around 85% of the people
intoxicated on marijuana WERE ALSO INTOXICATED ON ALCOHOL. For
people only intoxicated on marijuana, the rate was much lower than
for alcohol alone. This finding has been supported by other
research using completely different methods. For example, an
economic analysis of the effects of decriminalization on marijuana
usage found that states that had reduced penalties for marijuana
possession experienced a rise in marijuana use and a decline in
alcohol use with the result that fatal highway accidents decreased.
This would suggest that, far from causing "carnage", legal
marijuana might actually save lives.
Along with another misconception, that Marijuana could be lethal in large doses.
12. No one has ever died of a marijuana overdose
This is true. It was put in to see if you are paying
attention. Animal tests have revealed that extremely high doses of
cannabinoids are needed to have lethal effect. This has led
scientists to conclude that the ratio of the amount of cannabinoids
necessary to get a person intoxicated (i.e., stoned) relative to
the amount necessary to kill them is 1 to 40,000. In other words,
to overdose, you would have to consume 40,000 times as much
marijuana as you needed to get stoned. In contrast, the ratio for
alcohol varies between 1 to 4 and 1 to 10. It is easy to see how
upwards of 5000 people die from alcohol overdoses every year and no
one EVER dies of marijuana overdoses.
While I won't argue any points, I'm just going to leave it off here. Honestly consider what you're doing to your body and it's effects.
-TC