Half-life and redistribution
Gives you the number that predicts how long a drug stays — and a common exam calculation.
Where this fitsConnects to 2 · Unlocks 1
- Connects to
- Metabolism and excretion: the liver and kidneys — Metabolism and excretion set the half-life.
- Distribution: membranes, protein binding and barriers — Redistribution into tissues can end effects before elimination does.
- Unlocks
- Therapeutic window, steady state, washout — and tolerance — Half-life sets time to steady state and washout.
Orient
Half-life (T½): how long for the plasma level of drug to fall by 50%. For most drugs, enzymes metabolize a consistent proportion of circulating drug within the same amount of time – first-order kinetics. Some drugs (alcohol) metabolize a consistent amount regardless of dose – zero-order kinetics. Redistribution into tissues complicates the picture.
Course terms
Your professor's wording, word for word from the slides. Learn the term and this definition — the explanation underneath is only there to help it make sense.
- Half-life (T½)
“How long for the plasma level of drug to fall by 50% = half-life”Slides 04, p. 44
Related to duration of effects, but doesn't determine it on its own.
- First-order kinetics
“enzymes metabolize consistent proportion of circulating drug within the same amount of time – first-order kinetics”Slides 04, p. 44
The same fraction is cleared each half-life.
- Zero-order kinetics
“metabolize a consistent amount regardless of dose – zero-order kinetics”Slides 04, p. 44
Alcohol — “one drink per hour”.
- Redistribution
“Drug redistributes into tissues to reach equilibrium – causes rapid drop in plasma levels following first two hours (for IV)”Slides 04, p. 45
Once redistributed out of the brain, psychoactive effects are greatly diminished.
From your course: Course slides 04, pp. 44–45
Tip: the ← and → keys move through the lesson.
Full lesson: Half-life and redistribution