Charlene DeGidio never smoked marijuana in the 1960s, or afterward. But a year ago, after medications failed to relieve the pain in her legs and feet, a doctor suggested that the Adna, Wash., retiree try the drug.

Ms. DeGidio, 69 years old, bought candy with marijuana mixed in. It worked in easing her neuropathic pain, for which doctors haven't been able to pinpoint a cause, she says. Now, Ms. DeGidio, who had previously tried without success other drugs including Neurontin and lidocaine patches, nibbles marijuana-laced peppermint...