Enlarge (credit: California Medical Cannabis Initiative) BOSTON—Stacy Gruber of Harvard Medical School laid out the numbers: 28 states and the District of Columbia have medical marijuana laws, 17 others allow some cannabis-based products, and eight states now allow recreational use. The US has turned into a grand experiment on the medicinal use of pot, even as the federal government's classification of the drug makes it extremely difficult to do good research on it. But that doesn't mean research isn't getting done.