therapeutic index
Ratio between toxic dose and therapeutic dose of a medication, indicating safety margin.
Full Definition
The therapeutic index is a measure of drug safety calculated as the ratio between the dose that causes toxicity and the dose that produces the desired therapeutic effect. Medications with a narrow therapeutic index, such as lithium, require careful monitoring because small increases in dose can lead to toxicity, while small decreases may result in loss of efficacy. Drugs with a wide therapeutic index have a larger safety margin. In psychiatry, understanding therapeutic index is crucial for medications like anticonvulsants used as mood stabilizers, antipsychotics, and tricyclic antidepressants, where therapeutic drug monitoring may be necessary.
Usage
Usage note: Distinguish from 'therapeutic window' and 'therapeutic range'; critical concept for high-risk medications.
In Context
- "Lithium has a narrow therapeutic index, requiring regular serum level monitoring to prevent toxicity." — Medication education material
- "The therapeutic index of this antipsychotic allows for more flexible dosing without significant safety concerns." — Pharmacology consultation note