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Nicotine tests collect samples from either the hair, urine, saliva, or blood. Most nicotine tests don't actually test for nicotine. Instead, they look for one of its by-products: cotinine.

"In the body, nicotine is broken into its derivatives, the most important being cotinine," says Anis Rehman, MD, an assistant professor in the department of internal medicine at Southern Illinois University. "Cotinine usually lasts much longer than nicotine in the body. Therefore, most smoking tests look for cotinine."