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A ratio that compares the blood concentration at which a drug becomes toxic and the concentration at which the drug is effective. The larger the therapeutic index (TI), the safer the drug is. If the TI is small (the difference between the two concentrations is very small), the drug must be dosed carefully and the person receiving the drug should be monitored closely for any signs of drug toxicity.
If we plot a curve between plasma drug concentration and per cent of people then the desired effect in 50 per cent of people is called as- Effective dose in 50 % or ED50. and the dose of that particular drug which crosses the safety margin and starts to show side effect to serious lethal effect is called as LD50. LD50 is basically used for animals and for human beings it is termed as TD50 and toxic dose in 50 per cent of the population. so we can say that the therapeutic index is-
TI= Does that produces toxicity in 50 per cent of people divided by the dose that produces an effective clinical response in 50 per cent of people.
The therapeutic dose can also be stated as the distance between ED50 and LD50. a drug which has a value of therapeutic index more than 01 is considered as a safe and the drug having TI value less than 01 is considered as toxic.
The example of high therapeutic index drugs are- penicillin, Diazepam and Atenolol
Example of low therapeutic index drugs are- warfarin
lithium
digoxin
phenytoin
gentamicin
amphotericin
5-fluorouracil
zidovudine
In short - Therapeutic index is an expression of the relative toxicity as compared to the efficacy of a given compound. In animal studies, it can be computed as the ratio of the median lethal dose (LD50) to the median effective dose (ED50). In humans, it is frequently expressed as the ratio of the median toxic dose (TD50) to the ED50.
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