People have been using marijuana in one capacity or another for thousands of years, and yet scientists don't know that much about it. That's because, since 1999, the government has had an extra regulatory hoop for researchers to jump through — coincidentally, just as interest and funding was falling into place. Because of how marijuana was classified by the government (as a Schedule 1 drug, the "most dangerous" kind), studies about it would have to go through an additional Public Health Service review. Drugs classified this way, including heroin and ecstasy, are subject to abuse and are considered by the government to be difficult to use safely, even in a medical capacity. So regulators wanted to ensure that any study on these drugs was being done for the right reasons: To improve people's lives, and without too much risk. Many studies intending to show that marijuana could have positive effects on certain conditions were often rejected; even those that were approved would be delayed by years.