Of all the arguments that have been used to demonize marijuana, few have been more powerful than that of the "gateway effect": the notion that while marijuana itself may not be especially dangerous, it ineluctably leads to harder drugs like heroin and cocaine. Scientists abandoned the idea: as far back as 1999, in a report commissioned by Congress to look at the possible dangers of medical marijuana, the Institute of Medicine of the National Academy of Sciences wrote: "There is no conclusive evidence that the drug effects of marijuana are causally linked to the subsequent abuse of other illicit drugs."