The American Council for Drug Education reports marijuana, also known as THC for its active ingredient of delta-9-tetrahydrocannabinol, as the most commonly used illegal drug. To help increase productivity and fight the war on drugs, many employers, parents and schools routinely test for drugs through urine, blood or hair drug screens. These drug tests detect the metabolites of drugs--the substances produced when the body breaks down the drug. This allows for false-positive test results triggered by the metabolites of certain prescription and nonprescription medications, which occurs in approximately one to 2.5 percent of cases, according to the NetWellness Consumer Health Information.
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Dronabinol consists of the chemical delta-9-tetrahydrocannabinol, the active ingredient found in marijuana. Therefore, this medication produces a positive result when testing for THC. Doctors prescribe dronabinol to stimulate the appetite in patients with AIDS anorexia as well as to treat nausea and vomiting triggered by chemotherapy treatments. According to the Oregon State College of Pharmacy, doctors continue to perform additional studies testing the use of dronabinol in treating symptoms of multiple sclerosis, migraine and glaucoma.
Ibuprofen, classified as a nonsteroidal anti-inflammatory medication, effectively relieves pain and reduces swelling and stiffness. Both prescription and nonprescription medications contain ibuprofen, which often occurs in combinations with other drug such as in some cough and cold medications. In the past, ibuprofen proved to cause false-positive test results; however, the National Organization for the Reform of Marijuana Laws reports that this problem has been resolved.
Ketoprofen, similar to ibuprofen, relieves pain, swelling and stiffness. Often prescribed to treat rheumatoid arthritis--swelling of the joints--the chemical structure of ketoprofen resembles that of ibuprofen. This means the metabolites look similar and can cause the same interference with drug tests for THC.
Naproxen, also classified as a nonsteroidal anti-inflammatory drug, treats inflammation, swelling, stiffness and pain in joints. Available in prescription and nonprescription strength, naproxen can trigger a false-positive result on a drug test, according to NetWellness Consumer Health Information.
Promethazine, a prescription medication, blocks the effects of histamine, a substance produced and released in the body during an immune response. Doctors prescribe promethazine for many types of conditions. It relieves the symptoms of allergies, including runny nose, watery eyes and skin reactions. Promethazine also works as an anti-emetic, controlling the symptoms of nausea and vomiting. Because promethazine causes drowsiness, doctors often use it before and after surgery to sedate patients. Medications containing promethazine may produce false-positive results in THC drug tests.