This shouldn't be news. Researchers have known for decades that marijuana is a much safer substance than heroin -- and nearly all other psychoactive drugs, for that matter. Heroin overdoses kill thousands of people each year, while marijuana has no known toxic dose. It's considerably less habit-forming than heroin, alcohol, nicotine and other drugs. And medical marijuana treatments hold a lot of potential -- particularly as an alternative to dangerous prescription painkillers.
As far back as 1972, a federal report commissioned by President Richard Nixon recommended that marijuana use and possession be decriminalized, arguing that "neither the marihuana [sic] user nor the drug itself can be said to constitute a danger to public safety."
But Nixon ignored the report's findings. Marijuana was left on Schedule 1 of the Controlled Substances Act, the most restrictive category, reserved for drugs that "have no currently accepted medical use in the United States, a lack of accepted safety for use under medical supervision, and a high potential for abuse." Other Schedule 1 drugs include heroin and LSD.
In recent years as public opinion on marijuana has shifted and the science on the drug becomes more settled and accepted, the DEA has stuck with its anti-marijuana stance. Former DEA administrator Michelle Leonhart, who resigned under a cloud of scandal and managerial lapses earlier this year, famously refused to say whether marijuana was less dangerous than crack or heroin in a 2014 congressional hearing. The exchange drew widespread ridicule.