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 (Ath) 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, Ath and F are related by the formula, Ath = (N − F)/N. At adherence rates below Ath 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 Ath applicable to all drugs.