How Dangerous is Marijuana?

"Tetrahydrocannabinol is a very safe drug. Laboratory animals (rats, mice, dogs, monkeys) can tolerate doses of up to 1,000 mg/kg (milligrams per kilogram). This would be equivalent to a 70 kg person swallowing 70 grams of the drug—about 5,000 times more than is required to produce a high. Despite the widespread illicit use of cannabis there are very few if any instances of people dying from an overdose.

Safety of Medical Cannabis During Treatment

"Cannabinoids have a favourable drug safety profile. Acute fatal cases due to cannabis use in humans have not been substantiated, and median lethal doses of THC in animals have been extrapolated to several grams per kilogram of body weight. Cannabinoids are usually well tolerated in animal studies and do not produce the generalized toxic effects of most conventional chemotherapeutic agents. For example, in a 2-year administration of high oral doses of THC to rats and mice, no marked histopathological alterations in the brain and other organs were found.