drug reservoir

Some parts of the body act like a pantry, stocking away drug and releasing it gradually over time. A drug reservoir is a tissue or compartment that accumulates drug and then slowly returns it to the circulation, prolonging the drug's stay and sometimes its effect.

Common reservoirs include fat (for lipid-soluble drugs such as anaesthetics and some lipophilic agents), plasma proteins (a fast-exchanging reservoir), muscle and bone (which can sequester tetracyclines or heavy metals like lead), and even transcellular fluids. The stored drug is generally inactive while held, but acts as a slow-release supply: as plasma concentration falls, the reservoir feeds drug back into the blood.

Reservoirs can be helpful or harmful. They can smooth out blood levels and extend duration, but they can also cause unexpectedly long persistence and, when a reservoir is suddenly mobilised (for example, rapid weight loss releasing fat-stored drug), a delayed rise in blood concentration. Some long-acting formulations deliberately create a depot at the injection site.