A study finds chronic pain is the most common reason people give when they enrol in state-approved medical marijuana programs. That's followed by stiffness from multiple sclerosis and chemotherapy-related nausea.

The study didn't measure whether marijuana actually helped anyone with their problems. But the patients' reasons match up with what's known about the science of marijuana and its chemical components.

While the U.S. government has approved medicines based on compounds found in the plant, it considers marijuana illegal and imposes limits on research. That's led to states allowing some diseases and symptoms where rigorous science is lacking. Most of the evidence comes from studying pharmaceuticals based on marijuana ingredients, not from studies of smoked marijuana or edible forms.

Dementia and glaucoma, for example, are conditions where marijuana hasn't proved valuable, but some states include them. Many states allow Parkinson's disease or post-traumatic stress disorder where evidence is limited.

The analysis is based on 15 states that reported in 2016 the reasons cited for using marijuana. Researchers compared the reasons with a comprehensive review of the scientific evidence: a 2017 report from the National Academies of Sciences, Engineering and Medicine.

About 85 per cent of patients' reasons were supported by substantial or conclusive evidence in the National Academies report.

The study was published Monday in the journal Health Affairs.