Pharmacology · patient case question
Each of the following describes the mechanism of action of metformin EXCEPT one. Which is the EXCEPTION?
Patient case: Generalised deep pockets in type 2 diabetes
Patient: Male, 49 years old
Chief complaint: “I'm here for my check-up.”
History: Type 2 diabetes mellitus, hypertension, fatty liver. Medications: metformin, valsartan, atorvastatin. Wears a maxillary removable partial denture.
Findings: Generalised bleeding on probing with pocket depths of 5–8 mm in all four quadrants. Reports a persistently dry mouth.
Show answer and explanation
Correct answer: D. It increases glucagon production
- A. Suppressing hepatic gluconeogenesis is metformin's PRINCIPAL action, mediated through AMP-activated protein kinase. This is why it lowers fasting glucose so effectively.
- B. Reducing intestinal glucose absorption is a recognised, if secondary, contributor and helps explain the gastrointestinal side effects.
- C. Increasing peripheral insulin sensitivity, chiefly in skeletal muscle, is a genuine metformin mechanism — it improves glucose uptake without stimulating insulin secretion.
- D. CORRECT as the exception. Metformin DECREASES glucagon signalling rather than increasing it. Glucagon is the counter-regulatory hormone released in starvation to RAISE blood glucose by driving hepatic glycogenolysis and gluconeogenesis. A drug that increased glucagon would raise blood sugar, which is the opposite of what metformin does.
Note: Because metformin does not stimulate insulin release, it does not cause hypoglycaemia on its own — which is why a metformin-only patient does not need appointment timing adjusted around meals the way a sulphonylurea or insulin patient does. The serious rare adverse effect is lactic acidosis in renal impairment.