During week 6 , the average number of cigarettes smoked per day was lower for participants randomly assigned to cigarettes containing 2.4 , 1.3 , or 0.4 mg of nicotine per gram of tobacco ( 16.5 , 16.3 , and 14.9 cigarettes , respectively ) than for participants randomly assigned to their usual brand or to cigarettes containing 15.8 mg per gram ( 22.2 and 21.3 cigarettes , respectively ; P < 0.001 ) .