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Effect of compensation by smokers when smoking low tar cigarettes, as shown by mean blood levels of cotinine (with 95% confidence intervals) and related nicotine yield over time. 1=run-in to study; 2=entry; 3=at 2 months; 4=at 4 months; 5=at 6 months. Adapted from Frost et al (Thorax 1995;50: 1038-43)
