In pharmacokinetics, flip-flop phenomenon happens when a drug is released at a sustained rate instead of immediate release, such as sustained-release formulation vs. immediate-release formulation (tablet, IV).
In flip-flop kinetics, ka (absorption constant) is much slower than ke (elimination constant). This apparent difference shift the slope of log Cp vs time curve in which now the apparent part of ke looks much smaller than it is if the drug is administered intravenously or by immediate-release formulation. The part of downward curve becomes a reflection of actual ka while the upward part of the curve is the actual representation of ke. That "flip-flop" part of curve is the so-called flip-flop kinetics.
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