Half of patients who had taken cannabidiol (CBD) oil for four weeks had a positive urine test for THC, although none of them reported feeling high, a small study shows.

Seven of 14 patients taking part in a clinical trial to test the efficacy of CBD to treat anxiety ended up with a positive urine test for a major metabolite of delta-9-tetrahydrocannabinol (THC) even though the levels in the product they were given were far lower than the amount found in legal hemp-derived products.

Associate Professor Staci Gruber from Harvard Medical School, US, said it was something people should be aware of because testing positive may have legal and employment ramifications.

“People think because CBD is legally sold they won’t test positive in a urine screen,” she says.