A 500-mg dose of a drug has therapeutic efficacy for 6 h. If the half- life of the drug is 8 h, for how long would a 1gm dose be effective

The fact that the drug has therapeutic efficacy for 6 h has no direct relationship to its half-life--it simply means that the drug is above its minimal effective concentration for 6 h. Doubling the dose (to 1 g) means that the drug level will be above the minimum for a longer period. Because the elimination half-life is 8 h, 500 mg of the drug will remain in the body 8 h after a dose of 1 g. Thus, the total duration of effectiveness must be 8 + 6 = 14 hrs Ref:- kd tripathi; pg num:-56