How do you know if a drug requires periodic blood tests?

A drug may require periodic blood tests if it has a narrow therapeutic index, meaning the difference between effective and toxic doses is small. Additionally, if the medication is known to affect liver or kidney function, monitoring may be necessary to prevent adverse effects. Prescribing information, healthcare provider recommendations, and specific patient factors, such as age or existing health conditions, also determine the need for blood tests. Always consult a healthcare professional for personalized guidance.