Relationship Between Adherence Rate Threshold and Drug 'Forgiveness'.

The medication adherence rate (A) is the proportion of prescribed drug doses consumed within a given time period. It is often assumed that there is an adherence rate threshold (A ) at or above which the therapeutic effect of the medication is maintained. Drug forgiveness (F) is the number of consecutive doses that can be missed while still maintaining a therapeutic effect. At a given value for A, the therapeutic effect of the drug will be continuously maintained if there is no possibility of >F missed doses. Hence, for a once-daily drug prescribed for N days, A and F are related by the formula, A  = (N - F)/N. At adherence rates below A the probability of maintaining the therapeutic effect is equal to the probability of there being no instances of >F consecutive missed doses. Since F is a function of the duration of the drug effect (D) and D varies depending on the specific drug's pharmacokinetic/pharmacodynamic properties, there is no universal A applicable to all drugs.