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What does terminal elimination half life mean? Is it when half the drug degrades like terminal half life, or is it when enough half lives have passed that the drug is fully dissintegrated? I read: "fentanyl has a terminal elimination half-life of 219 minutes". Sorry for any typos I'm on a phone.
Terminal plasma half-life is the time required to divide the plasma concentration
by two after reaching pseudo-equilibrium, and not the time required to
eliminate half the administered dose. When the process of absorption is not a
limiting factor, half-life is a hybrid parameter controlled by plasma clearance
and extent of distribution. In contrast, when the process of absorption is a
limiting factor, the terminal half-life reflects rate and extent of absorption and
not the elimination process (flip-flop pharmacokinetics). The terminal half-life is
especially relevant to multiple dosing regimens, because it controls the degree of drug accumulation, concentration fluctuations and the time taken to reach
(full document attached).
As best as I can explain it, after peak plasma concentration of a drug, the drug starts exiting your system.
At this point, the drug is exiting fast (units of flow). At a point called the terminal phase, the drug starts exiting at a steady rate, and this gives the 'curve then straight line slope' look familiar to you if you've seen a drug in blood plasma concentration graph before. The terminal elimination half-life is the time it takes to lower the amount of drug in plasma by half (divide it by two), after it has reached steady-state absorption or steady-state plasma clearance/distribution. In other words, there are three limiting factors in how fast your body can absorb drugs, and this measure gives you a good baseline for re-dosing etc.
read the full text for a better explanation - it's 3.30am here!