If you’re being drug tested, chances are there’s quite a lot riding on the outcome, so you’d hope that the results are at least accurate and reliable. Yet according to a new study in the Journal of Psychopharmacology, it’s perfectly possible to fail a piss test for weed even if you haven’t smoked recently.

Most urine tests look for a compound called 11-nor-9-carboxy-tetrahydrocannabinol (THCCOOH), which is produced when the liver breaks down tetrahydrocannabinol (THC), the psychoactive compound in marijuana. However, the length of time that THCCOOH stays in the body for has never been fully determined.

A team of researchers therefore decided to recruit 70 weed smokers, all of whom were between the ages of 15 and 25, in order to investigate how long the compound could be detected in their urine. They were particularly interested in studying this age group as adolescents are believed to be the most frequent consumers of cannabis.

Participants were all allowed to smoke weed on the first day of the study, but were required to remain abstinent thereafter, with the researchers testing their urine every few days. A series of other biochemical tests were used to verify that none of the youngsters used any cannabis products while the study was in progress.

After a month with no marijuana, THCCOOH was still detectable in the urine of 40 percent of participants, while 20 percent contained levels that were sufficient to fail a standard federal drug test.

The researchers stopped testing participants’ pee after a month, and therefore did not determine the maximum time course for the detection of THCCOOH after smoking. Because of this, they claim that further studies are “essential for proper interpretation of drug testing outcomes in the workplace, drug treatment and forensic settings.”