Marijuana appears to be an effective treatment for chronic pain, nausea and symptoms of multiple sclerosis, concludes a new report from some of the nation’s top doctors and public health experts.

Commissioned by the National Academies of Sciences, Engineering and Medicine, the report says there’s “conclusive or substantial” research backing the effectiveness of cannabis for those three conditions. But it also warns of dangers from marijuana use: an increased risk of car crashes, lower birth weight babies and problems with memory and attention with heavy use. It also found strong connections between heavy cannabis use and the development of schizophrenia and other psychoses.

A wide variety of state and federal government agencies helped pay for the study, which included medical doctors, mental health practitioners and addiction specialists. The study’s authors said the increasing legalization of medical marijuana across the country makes this a “pivotal” moment in the national conversation. Legalization advocates hailed the...