Following absorption from its site of application, a drug reaches the bloodstream where it exerts its therapeutic effect. The concentration of a drug in the blood, or more specifically plasma, should be within a therapeutic window (or therapeutic range). This range should be above the minimum effective concentration (MEC) but below the minimum toxic concentration (MTC). If a drug concentration is below the MEC, no therapeutic effect will be seen and if the drug concentration is above the MTC, toxicity will occur. For drugs with a narrow therapeutic window, therapeutic drug levels do not differ greatly from levels associated with serious toxicity.(1)
There are a number of drugs with a narrow therapeutic window and these should be carefully considered when dispensed to patients. They include:
- Lithium
- Digoxin
- Phenytoin
- Ciclosporin
- Theophylline
- Carbamazepine.
