Two clinical trials with transdermal nicotine have shown that the 3-HC/cotinine ratio predicts response to transdermal nicotine such that faster metabolizers of nicotine (higher 3-HC/cotinine ratios) have lower quit rates, vs. slower nicotine metabolizers. Among abstainers in these trials, the 3-HC/cotinine ratio also predicts therapeutic levels of nicotine on transdermal nicotine, with faster metabolizers of nicotine exhibiting lower nicotine. Thus, faster metabolizers of nicotine may require higher nicotine doses to achieve the same therapeutic benefit from transdermal nicotine as do slow nicotine metabolizers.