Marijuana is one of the most commonly used recreational drugs, which continues to be illegal in most states. Screening drug tests, using samples of blood, urine or hair, are performed in a number of circumstances to test for the use of drugs. These tests detect the presence of the drugs themselves or their metabolites -- substances produced when the body breaks down the drugs. Urine tests are the most common screening tests for marijuana. They detect the presence of metabolites of delta-9-tetrahydrocannabinol (THC), which is the main active ingredient in marijuana. Several medications may cause positive screening tests for THC, including dronabinol, nonsteroidal anti-inflammatory drugs (NSAIDs), proton pump inhibitors (PPIs) and an anti-HIV medication called efavirenz (Sustiva).
Dronabinol (Marinol) is a pill containing a synthetic form of THC. It will therefore consistently produce a positive result during THC testing. Dronabinol is a prescription drug approved by the U.S. Food and Drug Administration (FDA) as an appetite stimulant for people with AIDS and a treatment of nausea and vomiting triggered by chemotherapy. Dronabinol may also be used off-label for other conditions, such as multiple sclerosis.
Nonsteroidal Anti-inflammatory Drugs
NSAIDs include a number of over-the-counter or prescription drugs, such as ibuprofen (Advil, Motrin), naproxen (Aleve) and ketoprofen (Orudis). These medications are used to reduce pain and swelling, as well as to treat fevers. They also occur in combination with other drugs in certain cough and cold medications. NSAIDS may sometimes cause false positive results on tests for THC. However, not all studies have reported that NSAIDs have this effect, according to a review article published in January 2008 in “Mayo Clinic Proceedings.” When present, positive results have been attributed to the ability of NSAIDs to interfere with an enzyme used in a common type of THC screening test.
Proton Pump Inhibitors
PPIs are a group of over-the-counter or prescription medications that reduce the amount of acid produced in the stomach. Omeprazole (Prilosec), esomeprazole (Nexium), pantoprazole (Protonix), lansoprazole (Prevacid) and rabeprazole (AcipHex) are common examples of PPIs. They are used to treat disorders that are worsened by high stomach acidity, such as gastroesophageal reflux disease or ulcers in the stomach or esophagus. As stated in the FDA label for pantoprazole, use of PPIs may produce a false positive result on urine screening tests for THC. The mechanism for this is unclear.
Efavirenz (Sustiva) is an antiretroviral medication used to treat HIV infection. According to the 2008 review article in “Mayo Clinic Proceedings,” a number of published reports have indicated that efavirenz can produce a false positive test for THC. It has been speculated that this is due to a metabolite of efavirenz interfering with one of the most common screening tests for THC.
Precautions and Next Steps
If you require a drug test, tell the person taking your sample about all of the prescription and nonprescription medications you are taking or have recently taken. This may be helpful if you have a positive result that is unexpected. If you test positive for THC on a screening test, request that the sample be verified with a more accurate test. Most false positive results on screening tests will produce negative results with this additional testing. However, if the positive test is due to dronabinol, the results will remain positive on additional tests since dronabinol contains THC.
Reviewed and revised by: Mary D. Daley, M.D.