"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. Moreover, THC treatment tended to increase survival and lower the incidence of primary tumours. Similarly, long-term epidemiological surveys, although scarce and difficult to design and interpret, usually show that neither patients under prolonged medical cannabinoid treatment nor regular cannabis smokers have marked alterations in a wide array of physiological, neurological and blood tests."
Guzman, Manuel, "Cannabinoids: Potential Anticancer Agents." Nature Reviews: Cancer (October 2003), p. 752.