Why are blood levels monitored for drugs such as phenytoin, digitalis, and lithium?

Blood levels for drugs such as phenytoin, digitalis, and lithium are monitored because they possess a relatively small therapeutic window.

This window represents the concentration range where a drug is effective; if the concentration falls below this level, the medication is ineffective, but if it rises above this level, the drug becomes toxic. Because individual responses to medication vary significantly due to factors such as genetics, age, and organ function, it can be difficult to determine the correct dosage for every patient. Monitoring plasma levels ensures that the drug remains within this narrow margin of safety, thereby maximizing therapeutic benefit while minimizing the risk of dangerous side effects.12