Medicine:Flip-flop kinetics

In pharmacokinetics, flip-flop occurs when a drug enters the blood stream at a slower rate than it is eliminated, i.e. when the k_a (absorption constant) is slower than k_e (elimination constant). These circumstances can occur with sustained-release formulations, depo IM injections, and some subcutaneous or intradermal injections. In the resulting slope of log plasma concentration (log Cp) vs time, the apparent k_e is determined by the k_a, and the apparent k_e is smaller than when the drug is administered intravenously or by immediate-release formulation.

The term 'flip-flop' indicates that the downward slope more closely represents k_a rather than k_e.