In adults, ibuprofen and hemp food products, including hemp oil supplements, cause false positives for THC, according to CVS.com. The University of North Carolina at Chapel Hill indicates that infants sometimes have a false positive THC test due to the chemicals in some baby soaps.
The screening tests used in employment and schools are more likely to return a false positive than the more precise tests where a lab collects and tests urine samples, according to CVS.com. Urine tests are often preferred as they are less invasive than other types of testing. The screening test alerts the potential employer if any of several drugs are in the urine above the threshold level. A false positive may prevent an applicant from getting a job or a student from playing a sport. The applicant has a better chance of protecting his rights if he knows he has taken legal substances that sometimes cause a false positive.
Hospitals also test babies for the presence of THC in their urine, according to the University of North Carolina at Chapel Hill. Positive tests may indicate a mother using marijuana while pregnant and, along with other indicators, may precipitate a call to child protective services. Research comparing the way the nursing staff collected the urine samples indicates the chemicals in several brands of baby soaps cause false positives.