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Drug screening is most often requested by employers for prospective employees. Many schools and parents, however, are also doing random drug tests when a teen is suspected of abusing drugs. Urine, blood or hair is tested to determine use of illegal drugs such as marijuana, cocaine, amphetamines, PCP and opiates. Although rare, false positive drug tests occur about 1 to 2.5 percent in urine tests due to a variety of medications and products.

Over-The-Counter Medications

Several over-the-counter medications such as common cold, allergy and sinus medications have been known to cause false positive drug tests. Ibuprofen in any form sometimes causes a false positive for marijuana use. Cold remedy medications with codeine have caused a false positive for use of morphine. Other medications to be aware of include Nyquil, Triaminic, Primatene, Bronkotabs, Vicks-44, Benadryl, Alka-seltzer plus, Contac, Sinuntab and Sudafed.

Prescription Medications

If you have a prescription for Valium, be aware that it sometimes causes a false positive result for PCP. Zoloft also has been shown to causes false positives, however, the substance for which it tested positive is unknown or unclear. Always write down any prescription medications you are using on your drug testing form.

Herbal Supplements

Valerian root, the supplement that is most often taken for anxiety or sleep problems, sometimes produces a false positive result for benzodiazepines. Other herbal supplements such as kava-kava and St. John's wort affect your body chemistry and sometimes lead to a false positive drug test.

Hemp Products

Most hemp-containing products are at risk for triggering a false positive for marijuana. Products include hemp-seed oil, hemp seasonings, hemp supplements and food items such as candy, cookies, bread, cooking oil and cheese. If you are accustomed to using or ingesting hemp-containing products, your best option is to avoid them in the days leading up to your drug test.