Says who
The National Academy of Sciennces' Institute of Medicine, for one: "there are patients with debilitating symptoms for whom smoked marijuana might provide relief. [...] Until a nonsmoked rapid-onset cannabinoid drug delivery system becomes available, we acknowledge that there is no clear alternative for people suffering from chronic conditions that might be relieved by smoking marijuana, such as pain or AIDS wasting." - Marijuana and Medicine: Assessing the Science Base (1999)
So smoking marijuana may be helpful with pain relief? So does drinking whiskey. But neither cures any disease, so neither is a medicine.