redistribution

Sometimes a drug's effect ends not because the body has destroyed or excreted it, but because the drug has simply moved house. Redistribution is the movement of a drug away from its site of action into other tissues — and for some drugs, this relocation is what switches the effect off.

It happens because the brain and other well-perfused organs receive a fat-soluble drug first, but as blood concentration falls, the drug flows down its gradient into slower-filling tissues such as muscle and fat. The classic example is the intravenous anaesthetic thiopental: it reaches the brain in seconds, but within minutes it redistributes into other tissues, the brain concentration drops, and the patient wakes — even though most of the dose is still in the body, awaiting slow metabolism.

Redistribution should not be confused with elimination. The drug is not yet gone; it is parked elsewhere and will re-enter the blood as concentrations fall. With repeated doses, the peripheral tissues fill up, redistribution can no longer end the effect quickly, and the drug's action becomes prolonged — an important practical caveat.