A patient with diabetes mellitus who was previously well controlled on glyburide now has fasting glucose levels around 180-200 mg/dL. Which medication may contribute to this hyperglycemia?

• A. Prednisone
• B. Phenelzine (Nardil)
• C. Atenolol (Tenormin)
• D. Allopurinol (Zyloprim)

Answer: (A)

Glucocorticoids cause hyperglycemia by increasing hepatic glucose production and reducing insulin sensitivity in muscle and fat. Prednisone promotes gluconeogenesis and counteracts insulin’s actions, so in a patient with diabetes controlled on glyburide it can push fasting glucose up to high levels like 180–200 mg/dL. This drug undermines the effect of sulfonylureas, which rely on boosting pancreatic insulin secretion, because the body's tissues become less responsive to insulin and more glucose is produced by the liver.

Other drugs listed don’t push glucose up as a primary effect. Phenelzine can affect mood and weight but isn’t a classic driver of new hyperglycemia. Atenolol may slightly alter glucose tolerance and can mask hypoglycemia, but it’s not a common cause of rising fasting glucose. Allopurinol has little impact on glucose metabolism.