minimum effective concentration

Below a certain level in the blood, a drug is present but not yet doing its job. The minimum effective concentration is the lowest plasma concentration at which the desired therapeutic effect just appears — the bottom rail of the therapeutic window.

MEC is a concentration, not a dose, which makes it the practical guide for dosing: clinicians design the dose, route, and dosing interval so that plasma levels rise above the MEC and stay there for as long as the effect is needed. The time between a drug crossing above its MEC and falling back below it is the duration of action.

MEC sits opposite the toxic concentration that forms the upper edge of the therapeutic window. Keeping a patient between them — above MEC for efficacy, below the toxic level for safety — is the whole aim of a maintenance dose at steady state. For drugs with a narrow gap, therapeutic drug monitoring confirms the level is staying in range.