Cotinine is the primary breakdown product of nicotine, and the numbers on a cotinine test tell you roughly how much nicotine has entered someone’s body over the past few days. In blood or saliva, daily smokers typically show cotinine levels between about 100 and 350 ng/mL, while people with no tobacco exposure usually fall below 1 ng/mL. But the gap between those extremes is where interpretation gets interesting, because the same cotinine number can mean very different things depending on the person’s genetics, what sample was tested, and even whether they are pregnant.
What Cotinine Is and Why It Gets Measured Instead of Nicotine
When nicotine enters your body, liver enzymes convert most of it into cotinine. This conversion happens primarily through an enzyme called CYP2A6, along with a few helper enzymes.1PubMed Central. Nicotine chemistry, metabolism, kinetics and biomarkers Nicotine itself disappears from the bloodstream quickly, with a half-life of about two hours. Cotinine hangs around much longer, with a half-life of roughly 16 to 18 hours.2PubMed Central. Biomarkers of Tobacco Exposure: Summary of an FDA-sponsored Public Workshop That longer window makes cotinine far more useful as a biomarker. A single blood draw or saliva swab can capture a person’s average nicotine exposure over the previous several days, rather than catching whatever spike or trough happened in the last couple of hours.
Cotinine can be measured in blood (serum), saliva, and urine. Each matrix produces different absolute numbers for the same person, so comparing a urine result to a blood reference range is a common source of confusion. Urine concentrations run highest, saliva falls in the middle, and serum tends to be lowest. The testing method matters too: clinical labs often use liquid chromatography with mass spectrometry for precision, while point-of-care screening kits use simpler immunoassay or colorimetric techniques that give semi-quantitative results.3PubMed Central. Diagnostic Methods for Detection of Cotinine Level in Tobacco Users: A Review
Typical Ranges for Smokers, Passive Smokers, and Nonsmokers
The clearest way to make sense of cotinine numbers is to see where they fall relative to each smoking category. In one study that measured both urine and saliva simultaneously, the differences were stark. Smokers averaged about 1,044 ng/mL in urine and 327 ng/mL in saliva. Passive smokers (people who lived or worked around smokers but did not smoke themselves) averaged about 37 ng/mL in urine and 18 ng/mL in saliva. Nonsmokers with minimal exposure averaged roughly 14 ng/mL in urine and 10 ng/mL in saliva.4PubMed Central. Assessment of cotinine in urine and saliva of smokers, passive smokers, and nonsmokers: Method validation using liquid chromatography and mass spectrometry
For blood (serum) cotinine, large epidemiological studies of daily smokers have found concentrations ranging from about 100 to 350 ng/mL.2PubMed Central. Biomarkers of Tobacco Exposure: Summary of an FDA-sponsored Public Workshop The wide range reflects differences in how many cigarettes people smoke per day, how deeply they inhale, and individual variation in metabolism. A 10-cigarette-a-day smoker might land near 100 ng/mL in serum, while a pack-a-day smoker could easily exceed 300 ng/mL.
Where Labs Draw the Line Between Smoker and Nonsmoker
If you are looking at a test result and trying to decide what it means in practical terms, the cut-point matters as much as the raw number. For serum cotinine, a widely cited analysis found that a threshold of 12 ng/mL best distinguished confirmed cigarette smokers from people who had never smoked regularly, achieving sensitivity and specificity both above 96%.5PubMed. Assessing smoking status in children, adolescents and adults: cotinine cut-points revisited But that single number is misleading if applied blindly. The same study found that the optimal cut-point shifted depending on the person’s home environment: 18 ng/mL worked best for people living with a smoker, while 5 ng/mL was more accurate for people in smoke-free homes. Social disadvantage also pushed the best cut-point higher, creating a gradient from about 8 ng/mL to 18 ng/mL.
For urine cotinine, which runs higher in absolute terms, one study proposed a tiered set of thresholds. A urine cotinine of 50 µg/L separated nonsmokers from passive smokers. At 550 µg/L, passive smokers could be distinguished from active smokers. And within the active-smoker group, 2,100 µg/L divided light smokers from heavy smokers.6PubMed. Estimation of urinary cotinine cut-off points distinguishing non-smokers, passive and active smokers These numbers are useful for population screening, but because urine concentration depends on hydration, individual results can bounce around from sample to sample more than serum or saliva values.
Why the Same Nicotine Intake Produces Different Cotinine Levels in Different People
Two people can smoke the same number of cigarettes and end up with meaningfully different cotinine readings. The biggest reason is genetic variation in CYP2A6, the liver enzyme responsible for converting nicotine into cotinine and then converting cotinine into its own downstream metabolite. People carry different versions of the CYP2A6 gene, and a systematic review of studies on this topic classified nicotine metabolism into four tiers based on how much enzyme activity a person’s genetic profile predicts: normal (more than 75% activity), intermediate (about 50–75%), slow (25–50%), and poor (less than 25%).7PubMed Central. Nicotine Metabolism Predicted by CYP2A6 Genotypes in Relation to Smoking Cessation: A Systematic Review A “slow” metabolizer keeps nicotine in their system longer and may need fewer cigarettes to feel satisfied, but their cotinine levels per cigarette can look different from those of a “normal” metabolizer who clears both nicotine and cotinine faster.
Racial and ethnic differences in cotinine levels are well documented, and they appear to trace partly back to CYP2A6 variation and partly to differences in smoking behavior. A study comparing Black and white smokers found that the total clearance of cotinine was significantly lower in Black participants, and their cotinine half-life ran longer (roughly 1,064 minutes versus 950 minutes). On top of that, nicotine intake per cigarette was about 30% higher in Black smokers.8JAMA. Nicotine Metabolism and Intake in Black and White Smokers The result is that for the same number of cigarettes smoked, Black smokers tend to show higher cotinine levels. Research into the mechanism found that in Black smokers, cotinine levels tracked closely with CYP2A6 enzyme activity, consistent with slower clearance being a primary driver of the difference rather than compensatory smoking behavior.9PubMed Central. Racial differences in the relationship between rate of nicotine metabolism and nicotine intake from cigarette smoking This matters clinically: if a lab applies a universal cut-point to flag “light” versus “heavy” smoking, it could miscategorize people based on how their body handles nicotine rather than how much tobacco they actually use.
Pregnancy Speeds Up Cotinine Clearance
Pregnant women clear cotinine from their bodies far faster than non-pregnant women. Pharmacokinetic modeling estimated that cotinine clearance during pregnancy is about 140% higher than in non-pregnant women, while nicotine clearance is about 60% higher.10PubMed Central. Physiologically Based Pharmacokinetic Modelling for Nicotine and Cotinine Clearance in Pregnant Women This means a pregnant woman who smokes the same amount as before pregnancy will show a substantially lower cotinine level on a test. The clinical risk has not decreased — the nicotine exposure is the same or nearly so — but the biomarker concentration has dropped. Anyone interpreting cotinine results during pregnancy needs to keep this in mind: the standard cut-points were derived from non-pregnant adults, and applying them directly to a pregnant patient could falsely reassure clinicians that smoking has stopped or decreased when it has not.
Kidney Disease and Other Medical Factors
The kidneys play a role in clearing both nicotine and cotinine from the body, so impaired kidney function can push cotinine levels higher. A pharmacokinetic study found that cotinine elimination was decreased in patients with kidney failure, and the nonrenal clearance of nicotine itself was roughly halved (about 661 mL/min in patients with severe kidney failure compared with 1,303 mL/min in healthy subjects).11PubMed. Pharmacokinetics of nicotine in kidney failure Even urine pH has an effect: acidifying the urine increased cotinine’s renal clearance by about 50% compared to baseline, though alkalinizing it made little difference.12PubMed. Cotinine disposition and effects The practical takeaway is that someone with chronic kidney disease who smokes may appear to have heavier tobacco exposure on a cotinine test than their actual intake would suggest.
Menthol Cigarettes and Altered Metabolism
Menthol, the flavoring compound in mentholated cigarettes, interferes with nicotine metabolism. In vitro work showed that menthol directly inhibits the liver enzyme that converts nicotine to cotinine.13PubMed. Inhibition of human liver microsomal (S)-nicotine oxidation by (-)-menthol and analogues A clinical study confirmed the effect in real smokers: switching to mentholated cigarettes significantly reduced the clearance of nicotine (about 1,289 mL/min versus 1,431 mL/min for non-mentholated cigarettes), with slower conversion to cotinine and slower glucuronide conjugation both contributing.14The Journal of Pharmacology and Experimental Therapeutics. Mentholated Cigarette Smoking Inhibits Nicotine Metabolism Because nicotine sticks around longer when metabolism is slowed, menthol smokers may maintain higher systemic nicotine exposure per cigarette. Their cotinine levels might actually appear lower, since less nicotine is being converted, but the nicotine itself is elevated. This creates a tricky interpretive gap: using cotinine alone to assess exposure in a menthol smoker could underestimate the actual nicotine burden.
E-Cigarettes and Other Nicotine Products
A common question is whether e-cigarette users show the same cotinine levels as traditional smokers. The answer depends heavily on how frequently and intensely they vape. E-cigarettes deliver nicotine effectively, though the pharmacokinetic profiles tend to be lower than those achieved with regular cigarettes.15PubMed Central. Nicotine and Cotinine Exposure from Electronic Cigarettes: A Population Approach That said, daily e-cigarette users may end up with serum cotinine levels comparable to or even slightly higher than those of combustible cigarette smokers. An NHANES analysis found that when looking at the subgroup who reported daily use, e-cigarette smokers had non-significantly higher serum cotinine than daily combustible cigarette smokers, suggesting that heavy vapers absorb plenty of nicotine.16Carcinogenesis. Serum cotinine levels and nicotine addiction potential of e-cigarettes: an NHANES analysis For both products, cotinine levels scaled linearly with the number of days of use. Urine cotinine measurements after a single use session showed similar peak levels and recovery across electronic cigarettes, combustible cigarettes, and smokeless tobacco.17Nicotine & Tobacco Research. Comparison of Urinary Biomarkers of Exposure in Humans Using Electronic Cigarettes, Combustible Cigarettes, and Smokeless Tobacco
What this means practically is that a positive cotinine test does not by itself tell you what nicotine source someone used. E-cigarettes, nicotine patches, nicotine gum, and smokeless tobacco all generate cotinine. If you are trying to verify whether someone has quit combustible cigarettes specifically, cotinine alone cannot answer that question.
Secondhand and Thirdhand Smoke Exposure
Cotinine testing is one of the most reliable ways to confirm that someone has been exposed to secondhand smoke, because people often underestimate or deny their exposure. In children, even low-level exposure registers. A study of secondhand smoke and pediatric healthcare visits used a serum cotinine threshold of 3 ng/mL to flag high secondhand smoke exposure and 0.05 ng/mL as the floor for any detectable exposure. Children with high exposure were roughly 3.5 times more likely to visit the emergency department and about 2.9 times more likely to have an overnight hospital stay compared with children whose cotinine was undetectable.18PubMed Central. Secondhand Smoke Exposure and Pediatric Healthcare Visits and Hospitalizations Among asthmatic children, both high and low secondhand smoke exposure categories were tied to more acute care visits and hospital admissions.
For children under five, age-appropriate serum cotinine cut-points for secondhand smoke have been studied, though the evidence is thinner than for adults. One study proposed 0.9 ng/mL for children under 12 and 0.6 ng/mL for those 12 and older, but no study has established race-, sex-, or age-specific cut-points specifically for the under-five group.19PLoS ONE. Serum cotinine cut-points for secondhand smoke exposure assessment in children under 5 years: A systemic review This gap matters because children metabolize nicotine differently from adults, and younger children may be exposed both through secondhand and thirdhand routes.
Thirdhand smoke is the residue that settles on surfaces, clothing, and dust after smoking occurs in a space. It is less intense than direct secondhand exposure but still measurable. A study assessing salivary cotinine found levels of 0.34 ng/mL in people exposed to both secondhand and thirdhand smoke, 0.22 ng/mL in people exposed to thirdhand smoke alone, and 0.11 ng/mL in unexposed individuals. Thirdhand-only exposure roughly doubled cotinine levels compared with the unexposed group.20PubMed. Third-hand exposure at homes: Assessment using salivary cotinine The numbers are small in absolute terms but they show that smoke residue in a home produces a detectable nicotine load even when nobody is actively smoking nearby.
Can Food Cause a Positive Cotinine Test?
This is a question that comes up surprisingly often, and the answer is: theoretically yes, but in practice, almost never at levels that matter. Nicotine is naturally present in nightshade vegetables, including tomatoes, potatoes, eggplant, and peppers, as well as in some teas. Tomatoes contain an average of about 7 ng of nicotine per gram of fresh weight, and potatoes about 15 ng/g. Tea leaves are more variable, with some instant teas reaching up to 285 ng/g.21PubMed. Dietary nicotine: a source of urinary cotinine Based on typical diets, an early calculation estimated that dietary nicotine could produce urinary cotinine concentrations somewhere between 0.6 and 6.2 ng/mL in a person who ate heavy amounts of these foods and drank a lot of tea.
That sounds like it could blur the line between a truly unexposed person and a passive smoker, but later work put the concern into better perspective. A follow-up analysis of fresh nightshade vegetables found nicotine content in the range of 2–7 µg/kg for fresh fruits, and estimated average daily dietary nicotine intake across several countries at roughly 1.4 µg per day, with even the 95th-percentile consumer reaching only about 2.25 µg per day.22PubMed. Determination of the nicotine content of various edible nightshades (Solanaceae) and their products and estimation of the associated dietary nicotine intake Another study specifically looked for cotinine in edible nightshade plants and could not detect it at concentrations above 0.1 µg/kg, concluding that the dietary contribution to salivary cotinine is not significant compared with even low-level tobacco smoke exposure.23Food Chemistry. The contribution of dietary nicotine and dietary cotinine to salivary cotinine levels as a nicotine biomarker So while an extremely rare scenario involving massive tea and potato consumption could nudge urinary cotinine above zero, it would not come close to mimicking a smoker or even a typical secondhand exposure level.
Beyond Blood and Urine: Hair and Nail Testing
Standard cotinine testing in blood, urine, or saliva captures exposure over the past few days. For situations that require a longer lookback, such as verifying smoking status over months, hair and nails offer an alternative. Nicotine accumulates in hair and nails as they grow, providing a record of chronic exposure that can span weeks to months depending on sample length. The noninvasive nature of the collection also makes these matrices especially useful for infants and small children, where drawing blood is more difficult.24Tobacco Induced Diseases. Hair and nail nicotine levels of mothers and their infants as valid biomarkers of exposure to intrauterine tobacco smoke Hair and nail testing is not routine in clinical settings, but it shows up in research on prenatal tobacco exposure and in some insurance or forensic contexts where long-term verification matters.
Using the Nicotine Metabolite Ratio to Personalize Treatment
Cotinine is not just a snapshot of exposure. It also feeds into a more sophisticated biomarker called the nicotine metabolite ratio, or NMR, which is the ratio of 3′-hydroxycotinine (a further breakdown product of cotinine) to cotinine itself. The NMR reflects how fast your body clears nicotine, which is largely determined by your CYP2A6 genetics.25PubMed Central. The Use of the Nicotine Metabolite Ratio as a Biomarker to Personalize Smoking Cessation Treatment: Current Evidence and Future Directions
This matters for quitting smoking. A randomized, placebo-controlled trial found that among “normal” metabolizers (people who clear nicotine relatively fast), the prescription drug varenicline was about twice as effective as the nicotine patch for quitting. But among “slow” metabolizers, there was no significant difference between the two treatments. The implication is that slow metabolizers can do just as well with the nicotine patch, which has fewer side effects, while normal metabolizers benefit from the stronger pharmacological approach.26The Lancet Respiratory Medicine. Nicotine metabolite ratio and cotinine as predictors of smoking cessation treatment response: a randomized, double-blind placebo-controlled trial The NMR is not yet part of standard clinical practice for most smokers, but it represents a direction the field is heading: using cotinine-related biomarkers not just to detect smoking but to tailor how you help someone stop.
Occupational Nicotine Exposure in Tobacco Farmworkers
Cotinine levels are not just about people who smoke or live with smokers. Tobacco farmworkers absorb nicotine directly through their skin while handling wet tobacco leaves, and this occupational exposure can produce cotinine levels on par with active smoking. Workers during the harvest period showed cotinine levels around 111 ng/mL, a range that squarely overlaps with light to moderate cigarette smokers, and they also showed roughly triple the DNA damage seen in controls.27PubMed. Impact of nicotine-induced green tobacco sickness on DNA damage and the relation with symptoms and alterations of redox status in tobacco farmers
The acute illness associated with this skin absorption is called green tobacco sickness, and it mimics nicotine poisoning: nausea, vomiting, dizziness, and weakness. A prospective study of 182 farmworkers found that among nonsmokers, each unit increase in the natural log of salivary cotinine roughly doubled the odds of developing the condition.28Nicotine & Tobacco Research. High levels of transdermal nicotine exposure produce green tobacco sickness in Latino farmworkers The risk varies by task: workers who handle leaves directly, especially in wet conditions, absorb far more nicotine than those working in shade-tobacco harvesting, where contact with the plant is less direct. Shade-tobacco workers in one study did not show significantly elevated cotinine levels and reported no green tobacco sickness symptoms.29PubMed. Cotinine levels and green tobacco sickness among shade-tobacco workers For anyone interpreting a farmworker’s cotinine test, the occupational route of exposure is important context: a positive result does not necessarily mean tobacco use.