Marijuana does raise the risk of getting schizophrenia and triggers heart attacks, according to the most significant study on the drug's effects to date. A federal advisory panel admitted cannabis can almost certainly ease chronic pain, and might help some people sleep.
But it dismisses most of the drug's other supposedly 'medical benefits' as unproven.
Crucially, the researchers concluded there is not enough research to say whether marijuana effectively treats epilepsy - one of the most widely-recognized reasons for cannabis prescriptions.
The report also casts doubt on using cannabis to treat cancers, irritable bowel syndrome, or certain symptoms of Parkinson's disease, or helping people beat addictions.
The experts called for a national effort to learn more about marijuana and its chemical cousins, including similarly acting compounds called cannabinoids.
In fact, the current lack of scientific information 'poses a public health risk,' said the report, released by the National Academies of Sciences, Engineering and Medicine.
Patients, health care professionals and policy makers need more evidence to make sound decisions, it said.
Several factors have limited research. While the federal government has approved some medicines containing ingredients found in marijuana, it still classifies marijuana as illegal and imposes restrictions on research. So scientists have to jump through bureaucratic hoops that some find daunting, the report said.
A federal focus on paying for studies of potential harms has also impeded research into possible health benefits, the report said. The range of marijuana products available for study has also been restricted, although the government is expanding the number of approved suppliers.
Twenty-eight states and the District of Columbia have legalized marijuana for a variety of medical uses, and eight of those states plus the district have also legalized it for recreational use.
