The Marijuana Policy Project says:
Clinton has expressed support for legal access to medical marijuana and more research into the medical benefits of marijuana.
In 2014, when asked about the legalization laws approved in Colorado and Washington, she said “states are the laboratories of democracy” and that she wants to see what happens in those states prior to taking a position in support or opposition to such laws.
During the October 13 Democratic presidential debate she was asked whether she has taken a position on state legalization laws now that a year has gone by, to which she replied, “No.” Instead, she expressed support for laws that allow legal access to medical marijuana, as well as concern about U.S. incarceration rates, noting that she does not believe people should be imprisoned for marijuana use. In an interview the following day, she expressed support for allowing states to adopt their own marijuana policies and said she would not want the federal government to interfere in them.
On November 7, 2015, Clinton said she supports reclassifying marijuana from Schedule I to Schedule II to remove barriers to researching its medical benefits.
It later quotes her as saying (on Jimmy Kimmel Live, March 24, 2016):
I think what the states are doing right now needs to be supported, and I absolutely support all the states that are moving toward medical marijuana, moving toward — absolutely — legalizing it for recreational use.
What I’ve said is let’s take it off the what’s called Schedule I and put it on a lower schedule so that we can actually do research about it. There’s some great evidence about what marijuana can do for people who are in cancer treatment, who have other kind of chronic diseases, who are suffering from intense pain. There’s great, great anecdotal evidence but I want us to start doing the research.