In addition to its use as a nutritional supplement, niacin (nicotinic acid or vitamin B3) is medically prescribed to treat hyperlipidemia and hypercholesterolemia. Use of niacin in low doses usually leads to few adverse drug reactions (ADRs); however, at larger doses, niacin can cause skin flushing, itching, and occasionally more serious effects. The 2005 annual report of the American Association of Poison Control Centers documented 3,109 reports of exposures to niacin. During 2006, the Rocky Mountain Poison and Drug Center (RMPDC) in Denver, Colorado, received multiple calls regarding ADRs after nonmedical use of niacin. A review of call records indicated various uses of niacin, including attempts to alter or mask results of urine drug tests, although no scientific evidence exists that ingestion of niacin can alter a drug test result. To determine the extent of niacin use in attempts to alter drug test results, reports to RMPDC of niacin ADRs were reviewed for the period January--September 2006. The results identified 18 persons who reported nonsuicidal, intentional, nonmedical reasons for using niacin, including eight who specified altering drug test results as their reason for using niacin. Ten other persons, among an additional 18 who offered no reason for niacin use, were categorized as possible users of niacin to try to alter drug test results because of their ages or the amount of niacin ingested. Clinicians, especially those whose patients include teens and young adults, should be aware of the potential use of niacin in attempts to defeat urine drug tests.