"A careful search of the literature and testimony of the nation's health officials has not revealed a single human fatality in the United States proven to have resulted solely from ingestion of marihuana. Experiments with the drug in monkeys demonstrated that the dose required for overdose death was enormous and for all practical purposes unachievable by humans smoking marihuana. This is in marked contrast to other substances in common use, most notably alcohol and barbiturate sleeping pills."
Shafer, Raymond P., et al, Marihuana: A Signal of Misunderstanding, Ch. III, (Washington DC: National Commission on Marihuana and Drug Abuse, 1972).