Cotinine is nicotine's major metabolite, which has a significantly longer half-life than nicotine. Cotinine measurement is often used to estimate a patient's tobacco/nicotine usage prior to quitting, and to confirm abstinence self-reports during follow-up. Cotinine is commonly measured in serum, urine, and saliva.

A breakdown product of nicotine that stays much longer in the blood than nicotine, and so can be used as a measurement of nicotine exposure, ETS exposure, or even nonsmoking compliance.

Cotinine is a metabolite of nicotine. Cotinine typically remains in the blood between 48 and 96 hours. The level of cotinine in the blood is proportionate to the amount of exposure to tobacco smoke, so it is a valuable indicator of tobacco smoke exposure, including secondary smoke.