True or False: A drug exhibiting a quantal response may be irreversible.

• A. True
• B. False
• C. Depends on patient
• D. Never irreversible

Answer: (A)

Quantal responses describe whether an effect occurs in an individual (yes or no) and are summarized as the fraction of individuals that respond at a given dose. The reversibility of that effect is about how long the effect lasts or whether the target is permanently changed, which is determined by the drug’s mechanism, not by the fact that the response is quantal. So it’s possible for a drug to produce a quantal response in which the effect is irreversible in the individual—for example, an irreversible enzyme inhibitor or permanent damage to a target can yield a lasting effect even though the population response at a given dose is all-or-none.