Aims: To assess the efficacy of nicotine replacement therapies (NRT) when the daily dose was adapted according to saliva cotinine concentrations., Design: Randomized, multi-centre, single-blind, controlled trial., Setting: Twenty-one smoking cessation clinics in France., Participants: A total of 310 smokers with medical comorbidities, motivated to quit, smoking ≥ 10 cigarettes/day, for whom smoking cessation was mandatory. NRT was administered for 3 months. The standard care group received nicotine patches with monthly dose decreases; buccal absorption NRT could be co-administered at the discretion of the investigator. In the dose adaptation group, the aim was a 100 ± 5% nicotine substitution with respect to smoking state based on the determination of saliva cotinine concentrations. NRT daily doses were prescribed according to the previous week's saliva cotinine concentrations in the dose adaptation group; saliva cotinine concentrations were not provided in the standard care group., Measurements: Prolonged abstinence rate (weeks 9-12, main outcome measure), point-prevalence and continuous abstinence rate, saliva cotinine concentration, NRT daily dose, craving for cigarettes., Findings: The median daily prescribed NRT dose was 30 and 31 mg/day in the first study week and 17.25 and 35.5 mg/day during weeks 9-12 in the standard care group and dose adaptation group, respectively. Saliva cotinine remained stable in the dose adaptation group and decreased in the standard care group (P < 0.01) by weeks 9-12. The cotinine substitution rate was significantly lower in the standard care group than in the dose adaptation group. Despite differences in NRT doses and cotinine substitution rates, prolonged (standard care group: 26.4%, dose adaptation group: 30.3%), continuous (standard care group: 8%, dose adaptation group: 12%) and point-prevalence abstinence rates were similar., Conclusions: In smokers with medical comorbidities and highly motivated to quit, adaptation of the nicotine replacement therapy daily dose according to saliva cotinine does not appear to be substantially superior to standard nicotine replacement therapy use., (© 2011 The Authors, Addiction © 2011 Society for the Study of Addiction.)