Eur J Clin Pharmacol (1985) 28:301-304 European Journal of Clinical Pharmacology © Springer-Verlag 1985 Steady-State Concentration of Cotinine as a Measure of Nicotine-Intake by Smokers R. L. Galeazzi 1, R Daenens 2, and M. Gugger 1 ~Department of Medicine, Inselspital Bern, University of Berne, Berne, Switzerland and 2Department of Toxicology, University of Louvain, Louvain, Belgium Summary. Measurement of plasma cotinine, the ma- jor metabolite of nicotine, is usually done to deter- mine nicotine-intake in smokers. Cotinine is used in- stead of nicotine because it has a much longer half-life than the mother substance and its plasma concentrations are therefore less dependent on the exact times of blood sampling. However, the lin- earity of the relationship between nicotine-intake and cotinine level in plasma has never been proven. Therefore cotinine was measured in 6 healthy volun- teers infused over 4 days with several doses of nico- tine i.v. up to 480 ~tg/kg/day. Cotinine concentra- tions in plasma were shown to be linearly and directly related to nicotine intake. The concentration of cotinine showed little variation during and for up to 2 h after the last dose of nicotine. Therefore, coti- nine can be used as an epidemiological marker of nicotine intake if it is measured around the time of the last cigarette of the day. Key words: nicotine, cotinine; steady-state plasma levels, smoking, cigarette consumption, nicotine in- fusion Nicotine, consumed primarily as inhaled cigarette smoke, is one of the most widely abused drugs in the world. Its deleterious effects on health are well known. As one attempt to reduce nicotine-intake, without affecting taste, industry has manipulated the design of cigarette filters, but such manoeuvres may not produce the desired reduction in the level of nic- otine in the smoker [1]. The discrepancy between the- ory and practice emphasizes the need for an objec- tive measure of true nicotine intake by smokers. The deleterious effects of cigarette smoking on health depend mainly on the amount of tar reaching the lung and the amount of nicotine reaching the cir- culation. As the tar-to-nicotine ratio is quite constant in each brand of cigarette, determination of nicotine- intake could yield most of the data necessary for study of the epidemiology of smoking. Nicotine, however, has a short half-life [2] in the body, in the range of only 2 h [3], so that its concen- tration in blood is sensitive to time of blood sampling and is primarily a measure of the last few cigarettes smoked. This is of great concern as individuals smoke with different itensities at different times. What is needed is a measure of average cumulative nicotine intake over a suitable period of time, e.g. at least the last day. Cotinine, the principal metabolite of nicotine [2], has a much longer half-life than nicotine around 17 h [4]. The concentration of cotinine in plasma has therefore been used recently [1, 5-7] as a marker of nicotine consumption. However, the inference of nicotine-intake from cotinine concentration can only be made if the relation between the 2 variables is a direct linear proportionality, i.e. metabolism of nico- tine and subsequent elimination of cotinine do not vary with concentration. In addition, nicotine intake would have to be fairly constant over at least three days, so that a steady-state of cotinine can be achieved. The present study was performed to test the hy- pothesis of a direct linear proportionality between nicotine intake and steady state cotinine. Material and Methods Six healthy volunteers, 2 females and 4 males, were studied. Their mean age was 23.5years (range 20-28years) and mean weight 61kg (range 52-66 kg). The study protocol was approved by the