Calculate the therapeutic index (TI) for a drug with a lowest effective dose of 20 mg and a lowest toxic dose of 200 mg. Based on the TI, how would you characterize prescribing safety?

• A. TI = 220; which indicates an exceptionally broad safety margin and no need to monitor the patient for toxicity during therapy.
• B. TI cannot be determined without information on the drug’s half-life and volume of distribution since those parameters define safety, not ratios of doses.
• C. TI = 10; the drug has a moderately wide safety margin making it relatively safe to dose compared to drugs with lower TI values.
• D. TI = 0.1; the drug is highly unsafe because the toxic dose is much lower than the effective dose, requiring extreme caution and monitoring.

Answer: (C)