Systematic review and meta-analysis investigating nicotine, cotinine, and carbon monoxide exposures in people who both smoke and use nicotine replacement therapy.

Abstract

Smoking is a major international public health problem. To determine effects of concurrent smoking and nicotine replacement therapy (NRT) use on reported heaviness of smoking, nicotine (cotinine) body fluid, and exhaled air carbon monoxide (CO) concentrations. Systematic review and meta-analysis of randomised controlled trials (RCTs) that test interventions permitting concurrent NRT use and smoking, and comparing, within participants, outcomes when smoking with those when smoking and using NRT concurrently. In total, 29 studies were included in the review. Meta-analysis of nine showed that, compared to when solely smoking, fewer cigarettes were smoked daily when NRT was used (mean difference during concurrent smoking and NRT use, -2.06 CPD [95% confidence interval {CI} = -3.06 to -1.07, P<0.0001]). Meta-analysis of seven studies revealed a non-significant reduction in exhaled CO during concurrent smoking and NRT use (mean difference, -0.58 ppm [95% CI = -2.18 to 1.03, P = 0.48]), but in the three studies that tested NRT 'preloading', a similar reduction in exhaled CO was statistically significant (mean difference, -2.54 ppm CO [95% CI = -4.14 to -0.95, P = 0.002]). Overall, 11 studies reported cotinine concentrations, but meta-analysis was not possible due to data reporting heterogeneity; of these, seven reported lower cotinine concentrations with concurrent NRT and smoking, four reported no differences, and none reported higher concentrations. People who smoke but who also use NRT reported smoking less heavily than when solely smoking, and when NRT was used as 'preloading' this reported smoking reduction was biochemically confirmed. There was no evidence concurrent smoking and NRT use resulted in greater nicotine exposure than smoking.