Question 199

Question:

Four drugs produce the same therapeutic effect by the same mechanism, but each has a unique toxicity. When used at a maximally effective therapeutic dose, the drug with which of the following sets of characteristics is most likely to have the highest incidence of toxicity?

Correct Answer: B.  Median Effective Dose 1x, Toxic Dose/Effective Dose 2.1

The closer the toxic dose is to the effective dose, the narrower the therapeutic window. Therefore the ratio of toxic/effective dose is what matters. If it is closer to 1, it has the highest risk of toxicity at effective doses, regardless of what the median effective dose actually is. Examples of drugs that have narrow therapeutic windows include lithium and digoxin.