I can attest to the pain-relieving properties of marijuana. Not a myth. Believe me, if it had just been wishful thinking I'd have known right away.
My back pain from spinal stenosis was intense and unrelenting, and unresponsive to chiropractic and physical therapy, as well as steroid injections. OTC pain meds did nothing and my digestive system couldn't handle opiates. Marijuana allowed me to drastically cut back on the conventional painkillers (oxycodone, vicodin, percoset, et al). The downside, as John noted above, is you can't get high from marijuana in this context.
I agree that there are better ways to ingest it than smoking. My preferred method is tea. That was only economically feasible, though, because I grew my own plants on the back porch. Store-bought pot is too expensive for brewing tea. You can, however, buy a tincture product that works very well. You just put a few drops into a cup of, say, peppermint tea. That addresses the most significant downside to eating/drinking the stuff: the difficulty of gauging dosage.
My son-in-law has cancer in his spine and kidneys. He, too, swears by the pain-relief he gets from marijuana, although he also takes morphine and oxycodone so it's hard to know how much the pot contributes.
We are lucky to live in a state where marijuana is legal.
Back to the topic. I asked my doctor the very same question that started this thread: what options are there other than opiates? His answer surprised and dismayed me. "None", he said. At one point I did get an injection of something they described as "super tylenol". That was so I could sit still long enough for an EKG. It worked. I jokingly asked if they'd send me home with a couple syringes of the stuff, but they said it's dangerous and they don't like to use it.
So there you have it. Nothing is anywhere nearly as effective as opiates. Aleve (Naproxin) is probably the most effective of the OTC medications, but I'm sure that kills your kidneys or something.