In pharmacokinetics, flip-flop occurs when a drug enters the blood stream at a slower rate than it is eliminated, i.e. when the ka (absorption constant) is slower than ke (elimination constant).
These circumstances can occur with sustained-release formulations, depo IM injections, and some subcutaneous or intradermal injections.[1] In the resulting slope of log plasma concentration (log Cp) vs time, the apparent ke is determined by the ka, and the apparent ke 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 ka rather than ke.