A new marijuana study joins a limited record of scientific knowledge about the harms and benefits of pot.
The research published Wednesday is the first rigorous test of a marijuana compound in treating a certain form of severe epilepsy. It found that an ingredient of marijuana — one that doesn't give pot smokers a high — reduced the number of seizures in children.
In the U.S., more than two dozen states allow medical use of marijuana. Federal drug regulators have not approved marijuana itself, but they have allowed man-made, chemically related medicines to treat loss of appetite in people with AIDS, and nausea and vomiting caused by cancer therapy.

A marijuana extract is sold in Britain for nerve pain and other problems from multiple sclerosis.
In January, a U.S. advisory committee concluded that the lack of scientific information about marijuana and its chemical cousins, called cannabinoids, poses a risk to public health. The experts called for a national effort to learn more.
In a report from the National Academies of Sciences, Engineering and Medicine, they also rounded up what is known. Here are some of its conclusions.
There's strong evidence that marijuana or cannabinoids:
— Can treat chronic pain in adults
— Can ease nausea and vomiting from chemotherapy
— Can treat muscle stiffness and spasms in multiple sclerosis as measured by what patients say, but less strong evidence if the changes are measured by doctors
On the other hand, it also found that pot smoking may be linked to:
— Risk of developing schizophrenia and other causes of psychosis, with the highest risk among the most frequent users
— Risk of a traffic accident
— More frequent chronic bronchitis episodes from long-term use
— Lower birthweight in offspring of female users
There's some evidence that pot or cannabinoids may:
— Improve short-term sleep in people with some medical conditions
— Boost appetite and ease weight loss in people with HIV or AIDS
— Ease symptoms of post-traumatic stress disorder and improve outcomes after traumatic brain injury
Similarly, some evidence suggests pot use may be linked to:
— Triggering a heart attack
— An increased risk of developing a lung condition called chronic obstructive pulmonary disease
— Pregnancy complications when used by the mother
— Impaired school achievement and outcomes
— Increased suicidal thoughts and suicide attempts, especially among heavier users
— Risk of developing bipolar disorder, especially among regular users.
There's not enough evidence to know if marijuana or cannabinoids can:
—Treat cancer
— Ease symptoms of irritable bowel syndrome
— Help with movement problems associated with Parkinson's disease
— Improve mental health outcomes in people with schizophrenia
Similarly, there's not enough evidence to know if marijuana is linked to:
— Increased risk of heart attacks over time from chronic use
— Development of post-traumatic stress disorder
— Changes in the course or symptoms of depression disorders
— Development or worsening of asthma
— Accidents or injuries on the job