Efficacy and toxicity are two key aspects of drug response of clinical relevance.
Efficacy refers to the maximum beneficial or therapeutic response that a drug can produce, and is a measure of clinical effectiveness. It can be expressed in terms of the percentage of recipients who show a therapeutic response at a given, standard dose.
Toxicity refers to the extent to which a drug induces unwanted or harmful health effects, and may be expressed as the percentage of patients who show adverse side effects at a given dose. The optimal dose range for a drug is that at which efficacy is greatest and toxicity is lowest. In practice achieving an acceptable level of toxicity usually means that efficacy is compromised and in some cases, even the safest drugs can produce severe side-effects. Extreme drug responders within a patient population are of concern. These include non-responders in whom the drug is ineffective, and adverse responders in whom the drug causes major harmful side-effects. If it was possible to identify these individuals in advance, treatments could be improved; the non-responders could receive an increased dose or alternative medication, and the adverse responders could receive either a greatly decreased dose or an alternative form of medication.
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