Limited evidence suggests that cannabis may alleviate neuropathic pain in some patients, but insufficient evidence exists for other types of chronic pain, according to a study reported yesterday in the Annals of Internal Medicine. The study reviewed the benefits of plant-based cannabis preparations for treating chronic pain in adults and the harms of cannabis use in chronic pain and general adult populations. Among general populations, limited evidence suggests that cannabis is associated with an increased risk for adverse mental health effects, the authors said. According to the study, 28 states and the District of Columbia have legalized cannabis for medical purposes.