Randomised, parallel, single-blind clinical trial in a primary health care setting in Majorca (Spain). Smokers in contemplation or pre-contemplation phase will be included in the study. Exclusion criteria: Smokers in preparation phase, subjects with a terminal illness or whose health status does not allow them to understand the study or complete the informed consent, and pregnant or breastfeeding women. The subjects will be randomly assigned to the control group (CG) or the intervention group (IG). The CG will receive brief advice, and the IG will receive brief advice together with a measurement of exhaled CO. There will be follow-up evaluations at 6 and 12months after inclusion. 471 subjects will be needed per group in order to detect a difference between groups≥5%. Primary outcome: sustained smoking cessation (at 6 and 12months) confirmed by urine cotinine test. Secondary outcomes: point smoking cessation at 6 and 12months both confirmed by urine cotinine analysis and self-reported, reduction in cigarette consumption, and variation in phase of smoking cessation.

CO-oximetry is an inexpensive, non-invasive, fast technique that requires little technical training; making it a technique for risk assessment in smokers that can be easily applied in primary care and, if proven effective, could serve as a reinforcement aid in smoking cessation intervention activities.