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The therapeutic index (TI; also referred to as therapeutic ratio) is a quantitative measurement of the relative safety of a drug with regard to risk of overdose.It is a comparison of the amount of a therapeutic agent that causes toxicity to the amount that causes the therapeutic effect. [1]
When a drug has a narrow therapeutic index, titration is especially important, because the range between the dose at which a drug is effective and the dose at which side effects occur is small. [2] Some examples of the types of drugs commonly requiring titration include insulin, anticonvulsants, blood thinners, anti-depressants, and sedatives.
Tricyclics have a narrow therapeutic index, i.e., the therapeutic dose is close to the toxic dose. [7] Factors that increase the risk of toxicity include advancing age, cardiac status, and concomitant use of other drugs. [8] However, serum drug levels are not useful for evaluating risk of arrhythmia or seizure in tricyclic overdose. [9]
Therapeutic drug monitoring (TDM) is a branch of clinical chemistry and clinical pharmacology that specializes in the measurement of medication levels in blood. Its main focus is on drugs with a narrow therapeutic range , i.e. drugs that can easily be under- or overdosed. [ 1 ]
TCAs, particularly nortriptyline, have a relatively narrow therapeutic index, which increase the chance of an overdose (both accidental and intentional). Symptoms of overdose include: irregular heartbeat , seizures , coma , confusion , hallucination , widened pupils, drowsiness , agitation , fever , low body temperature , stiff muscles and ...
A compound with a wide therapeutic index (greater than five) exerts its desired effect at a dose substantially below its toxic dose. Those with a narrow margin are more difficult to dose and administer, and may require therapeutic drug monitoring (examples are warfarin, some antiepileptics, aminoglycoside antibiotics).
The therapeutic index (TI) is used to quantify the risks and benefits of a certain drug. It describes the relationship between toxic dose and minimum effective dose, thus providing an important insight into the safety of a drug. [35] The Therapeutic Index is calculated using the following equation:
Cardiovascular agents generally have narrow therapeutic indices, implying that small differences in dose or blood concentration may give rise to adverse drug reactions. [45] Serious acute toxicity may result from accidental, intentional or iatrogenic overdose. [46] Therefore, patients need to be aware of any unusual and serious side effects.