Question 1

What is the minimum effective concentration (MEC)?

Accepted Answer

The minimum effective concentration (MEC) is the lowest plasma drug concentration that produces the desired pharmacological effect. It is the lower boundary of the therapeutic window — concentrations below the MEC are subtherapeutic and will not produce the desired clinical response. The MEC is drug-, effect-, and patient-specific: the same drug may have different MECs for different clinical endpoints (e.g., analgesic MEC vs anti-inflammatory MEC for an NSAID), and inter-individual variability in receptor sensitivity means the MEC varies across patients.

Question 2

How is the MEC related to the therapeutic window and Cmin?

Accepted Answer

The therapeutic window (also called therapeutic range) is bounded by the MEC on the lower end and the minimum toxic concentration (MTC) on the upper end. Trough levels (Cmin) measured in therapeutic drug monitoring are compared against the MEC to ensure the dosing interval maintains concentrations above the subtherapeutic threshold. For drugs with narrow therapeutic windows (e.g., digoxin, lithium, aminoglycosides), the MEC and MTC are close together, and small changes in clearance or dose can push concentrations below MEC or above MTC.

Question 3

How does the MEC relate to pharmacodynamic models?

Accepted Answer

In pharmacodynamic modelling, drug effect is often described by the sigmoid Emax model (Hill equation): E = Emax × C^n / (EC50^n + C^n), where EC50 is the concentration producing 50% of maximum effect. The MEC can be operationally defined as the concentration producing a minimum clinically meaningful effect (e.g., 10–20% of Emax). The MEC is therefore a clinical/empirical threshold rather than a fixed PD parameter — it represents the point at which the dose-response curve produces acceptable efficacy for a specific indication. It is particularly important in pain management, where insufficient analgesia below MEC is clinically relevant.

Question 4

How does the dosing interval affect time above MEC?

Accepted Answer

The fraction of the dosing interval that plasma concentrations remain above the MEC (t>MEC or T>MEC) determines the duration of pharmacological effect. For time-dependent pharmacodynamics (e.g., beta-lactam antibacterials, where effect depends on %T>MIC), maximising time above threshold is the key PK/PD driver, favouring more frequent dosing or extended infusions rather than high peak concentrations. For concentration-dependent drugs (e.g., aminoglycosides), achieving a high Cmax/MEC ratio is more important than prolonged time above threshold. Most analgesics have intermediate PD, where both the magnitude and duration of concentrations above MEC contribute to efficacy.

Minimum effective concentration

MEC within the therapeutic window

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