The amounts of THC and CBD in medical cannabis products often are different from what is stated on the label, according to a new study.

Researchers at Massachusetts General Hospital found some cannabis products that are advertised as not containing THC, the psychoactive ingredient in cannabis, in fact contain significant amounts. Many products advertised as containing CBD, an essential component of medical marijuana that does not cause a high, did not contain any.

The study analyzed urine samples from nearly 100 people enrolled in a clinical trial to look at the effect of medical marijuana on anxiety, depression, pain or insomnia, HealthDay reports. They found in about one-third of urine samples, no CBD was found, even though the products were supposed to contain mostly CBD or an equal amount of CBD and THC.

THC was found in almost 80% of the samples, including those from patients who thought they were using products only containing CBD.

“A lot of questions about the content of the products and their effects remain,” lead researcher Jodi M. Gilman, Ph.D., said in a news release. “Patients need more information about what’s in these products and what effects they can expect.”