"Last dance with Mary Jane. One more time to kill the pain."
Ok, admittedly I highly doubt that Tom Petty or his Heartbreakers had medicinal uses of marijuana to fight chronic pain in mind while writing Mary Jane's Last Dance, but all the same, emerging research seems like this might not have been far off.
With a growing number of US states legalizing medical marijuana evidence about the effectiveness of cannabis (the preferred term apparently when trying to distance the substance from connotations of 90's skaters with baggy jeans and long hair) on a number of ailments is mounting. According to CNN, the number 1 use, is to combat pain. When I initially read this I thought, "Yeah, ok, but narcotics also treat 'pain' but they don't help at all with fibromyalgia pain." However, an article from Extract says it seems that there may be some research to support THC and CBG (the key 'active ingredient' molecules in marijuana if you will) may actually help with fibro pain, perhaps because they are able to modulate pain signals from both peripheral nerves and the central nervous system.
This info comes with the big grain of salt that Extract, and all the studies I found reference to are published on sites interested in increasing the usage of medicinal marijuana, so, not unbiased sources. That being said, the article does link to a additional info on studies performed, at some reputable universities and published in legit medical journals, over the last decade or so. Study sizes did seem low, which makes it hard to draw true statistical significance from them. But, for those living in a state with legalized medical marijuana, it's an interesting consideration and might be worth talking to your doctor about.