Therapeutic index

The therapeutic range of a drug is the distance between its therapeutic dose and a dose that leads to a toxic effect. The greater the therapeutic range, the safer the medicinal product. This is generally expressed as a "therapeutic quotient", which is calculated as follows:

ED50 (median effective dose) is the dose at which the desired therapeutic effect occurs in 50 % of the individuals, and LD50 (median lethal dose) is the dose at which 50 % of the individuals die.

It should be noted here that the therapeutic quotient of LD50 and ED50 is in many cases not regarded as absolutely reliable for determining the therapeutic range, since the so-called effect curves of the ED and the LD can be differently steep and thus a false result for the hazard can emerge. It is more reliable to use the therapeutic index, a quotient of LD5 (lethal effect in 5% of the subjects) and ED95 (desirable effect in 95% of the subjects), but this may also give false results for dangerousness, albeit on a much smaller scale. Absolutely reliable statements on the therapeutic safety of a drug can probably only be obtained from the entire course of the dose-response and dose-lethality curves and not necessarily from the calculation of a quotient.

Drugs with a wide therapeutic range include, for example, glucocorticoids, penicillin, other β-lactam antibiotics, the cardiac glycoside strophanthin and the oral administration of diazepam. Drugs with a narrow therapeutic range include the digitalis cardiac glycosides, lithium and theophylline. When they are used, therefore, their effects must be carefully monitored, in some cases by laboratory testing of blood levels. Many narcotics are characterized by a narrow therapeutic range. Their effect, the depth of anaesthesia, must therefore be constantly monitored by the anaesthetist over time; see Anaesthesia (monitoring the patient).

Since the definition of the therapeutic quotient is based on the death of the treated person, the protective quotient was defined analogously with the toxic dose. The protective quotient represents a more realistic safety assessment for a drug approval, but uses the less unambiguous measure of a previously defined scale of toxic effects in each case.

See also

• drug monitoring
• Bioequivalence, bioavailability
• Pharmacodynamics is the study of the effect of drugs in the organism.
• Pharmacokinetics describes how quickly and to what extent after administration of a substance it subsequently appears in the blood plasma and in the various body tissues, and where and in what way it is excreted again.
• Adverse effects (old: side effects)