A Department of Justice/DEA brief from 1988 reads:
[A] marijuana smoker would have to consume 20,000 to 40,000 times as much marijuana as is contained in one marijuana cigarette. … A smoker would theoretically have to consume nearly 1,500 pounds of marijuana within about fifteen minutes to induce a lethal response.
Hence the urgent need for the federal government to classify it in the same category as heroin, i.e. "there is a lack of accepted safety for use of the drug", and more dangerous than cocaine or morphine. However, Richard Metzger warns: "[W]hereas you can see that it’s impossible for a human to OD on cannabis, the plant is HIGHLY toxic to dogs."