Half-life and redistribution
Gives you the number that predicts how long a drug stays — and a common exam calculation.
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- 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.
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- Therapeutic window, steady state, washout — and tolerance — Half-life sets time to steady state and washout.
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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.
Términos del curso
Las palabras de tu profesor, tal cual aparecen en las diapositivas. Aprende el término y esta definición; la explicación de abajo solo ayuda a entenderla.
- Half-life (T½)
“How long for the plasma level of drug to fall by 50% = half-life”Diapositivas 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”Diapositivas 04, p. 44
The same fraction is cleared each half-life.
- Zero-order kinetics
“metabolize a consistent amount regardless of dose – zero-order kinetics”Diapositivas 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)”Diapositivas 04, p. 45
Once redistributed out of the brain, psychoactive effects are greatly diminished.
De tu curso: Course slides 04, pp. 44–45
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