The therapeutic range is defined as the range of plasma drug concentration 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.

The therapeutic index is the ratio of the median lethal dose (LD50, the dose that kills 50% of the animals) to the median effective dose (ME50, the dose that produces a therapeutic effect in 50% of the animals).

Large therapeutic index means that there is big difference between the lethal and the effective dose of the drug which indicates that the drug is relatively safe. Drugs with large therapeutic index usually have wide therapeutic range.

However, small therapeutic index means that there is small difference between the lethal and the effective dose of the drug which indicates that the drug is relatively not safe. Drugs with small therapeutic index usually have narrow therapeutic range.