(NBC News) – There’s been little evidence on how well CBD works or guidance on how to safely use capsules, edibles or oils that contain cannabidiol.

Over the course of four weeks, 201 volunteers received either 5 mg per kilogram of body weight (approximately 2.3 mg per pound or the equivalent of 350 mg for a 154-pound individual) of oral CBD, or a placebo. The goal was to give them a typical amount that might be used by consumers. Participants also underwent weekly laboratory assessments.

While the vast majority of people in the trial were unaffected, 5% showed greatly elevated levels of the liver enzyme aminotransferase, a known marker of liver cell damage or inflammation, according to the study published Monday in JAMA Internal Medicine. Women appeared to be more vulnerable than men. The liver enzyme returned to normal within one or two weeks of stopping CBD.

Seven participants withdrew from the trial because they were showing clinical signs of potential drug-induced liver injury. (Read More)