FAQ 4: Considering Therapeutic Index & Lethal Dose; which one from the below conditions is applicable to be a relatively safer drug for the patients in terms of overdose toxicity & side effects?

1. Drugs with; “larger therapeutic index and lower lethal dose”.

2. Drugs with; “smaller therapeutic index and higher lethal dose”.

3. Drugs with; “equal therapeutic index and lethal dose”.

4. Drugs with; “larger therapeutic index and higher lethal dose”.

5. Drugs with; “smaller therapeutic index and lower lethal dose”.

Answer:
Right answer is the condition number 4. Drugs with; “larger therapeutic index and higher lethal dose”.

Why?
Because Therapeutic Index is the ratio of minimum effective dose to get therapeutic effect vs. minimum lethal dose to create toxicity.

Therapeutic Index (TI) = Lethal Dose (LD50)/ Effective Dose (ED50).

So larger ratio ensures larger therapeutic index to have significant diffetence between effective dose and lethal dose. Higher the lethal dose larger the therapeutic index which makes a drug more safe for the patient in terms of overdose toxicity and side effects. Most of the drugs has relatively larger therapeutic index except few and these drugs with narrow therapeutic index always leaves patients at a risk of overdose toxicity thats why needs frequent close monitoring of therapeutic drug level and the patient.

The list below mentions some examples of narrow therapeutic index drugs which needs close monitoring after use:
  • Warfarin.
  • Lithium.
  • Digoxin.
  • Phenytoin.
  • Gentamycin.
  • Amphotericin B.
  • 5-fluorouracil.
  • AZT (zidovudine)
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