Nicotine can be detected in the blood of people exposed to nicotine-containing products, whether from smoking cigarettes, chewing tobacco, environmental smoke, or second-hand smoke. However, nicotine breaks down in the body very rapidly, so nicotine levels are only measurable for a few hours after exposure.

Cotinine is produced by the body as it breaks down nicotine. It does not generally arise from other sources. Cotinine remains in the body for longer periods of time. Measuring cotinine levels in the blood has been used to determine whether people have been exposed to nicotine.

How long cotinine will be detectable in blood depends on how high the starting level is. The level of cotinine will drop by 50% in about 18 to 20 hours. For people exposed to small amounts of nicotine (i.e. light smokers) the cotinine level may be similar to the level seen from environmental exposure. For people with high amounts of nicotine exposure (i.e. heavy smokers) the cotinine level may be elevated for multiple days.