Hi and welcome to XXXXXXX
Thanks for the query.
It depends on test,but some NSAIDs( such as ibuprofenum),B12 vitamin or antiacid drugs can cause false positive results. Also, phenylpropanolamine
may interfere with test results. However,such false positive results are not common and if there are concerns about reliabilty of test result,this person should be sure that didnt consume marijuana/kanabis and marijuana similar products.
Wish you good health. Feel free to ask. regards