Which statement best describes flip-flop pharmacokinetics?

• A. The rate of elimination is slower than the rate of absorption.
• B. The rate of absorption is slower than the rate of elimination.
• C. Absorption and elimination rates are equal.
• D. Absorption is zero-order while elimination is first-order.

Answer: (B)

Flip-flop pharmacokinetics occurs when the slowest step in the drug’s disposition is absorption rather than elimination. When the absorption rate constant (ka) is smaller than the elimination rate constant (ke), the system’s apparent terminal decline is governed by absorption, not by elimination. In other words, the drug is absorbed slowly and enters the body gradually, while the body can eliminate it more quickly; the observed terminal half-life then reflects the slower absorption process. This is why the statement that absorption is slower than elimination best describes flip-flop PK.