The therapeutic range is defined as the range of plasma drug concentrations in which the probability of the desired therapeutic effect is high and the probability of the unacceptable toxicity is low. The lower limit of the therapeutic range is the minimum effective concentration (MEC) and the upper limit of the therapeutic range is the minimum toxic concentration (MTC).

The therapeutic range represents a range of drug conc in plasma and not at the site of action. This means that when the plasma conc of the drug is within the therapeutic range, the drug conc at the site of action (may be lower or higher than the plasma conc) produces the desired therapeutic effect.

The therapeutic range is different for different drugs. When the therapeutic range is wide, the drug is said to have a large therapeutic index, while when the therapeutic range is narrow, it is said that the drug has small therapeutic index. Drugs with small therapeutic index require careful selection of the dosing regimen to achieve therapeutic drug concentration.