Which of the following drugs have narrow therapeutic index: (A) Phenytoin, (B) Lithium, (C) Digoxin, (D) All of the above
Answer is D. Therapeutic index measures the safety of the drug. Wider therapeutic index safer is the drug. Hence, drugs like lithium, Phenytoin, Digoxin which have narrow therapeutic index can become either ineffective or toxic with small variations in the dosage.