text: i have not smoked marijuana in 1 year and a urine test showed a trace of thc, why? i do take alot of herbal vitamins for kidneys, immune system, etc would any of this affect the test ok here's one option;  thc - substances or conditions which can cause false positives dronabinol (marinol) ibuprofen; (advil, nuprin, motrin, excedrin ib etc) ketoprofen (orudis kt) kidney infection (kidney disease, diabetes) liver disease naproxen (aleve) promethazine (phenergan, promethegan) riboflavin (b2, hempseed oil)  here's another option;  sustiva is the hiv drug that has been most linked to false + urine tests for marijuana (sustiva also know as efavirenz=stocrine is some parts of the world). kh  here's another;  i worked in the field of mental health for 25+ years, and drug testing was a common reality. one of the facts which should be well-known to anyone in the field is that thc  gets stored in the fat cells, so if one is at all overweight it's likely that they have thc stored. if the person then exercises during the week of testing, it can result in the release of sufficient thc metabolites into the bloodstream to yield a positive test, since the test looks for metabolites, and not thc itself. so you won't get stoned when the metabolites are released into your bloodstream when you exercise, but the test will still detect them. it can take several years for these metabolites to be released and excreted from the body, so false positives in former cannabis users are not uncommon, and this  should be taken into consideration by test reporting agencies.  even normal activities of daily life and the concurrent level of metabolism can and will release thc metabolites into the bloodstream, and can produce a false positive.  these are the best i could find. i did a yahoo search for "thc false-positive urine test".  you might want to continue searching yahoo and google for the answer. i've sited my sources below. (btw, if i were you, i would talk to a doctor and have a physical done just to be sure everythings ok, as stated above, it could be a sign of other serious conditions)  sorry and good luck!