Metformin hydrochloride

How it works

It lowers blood glucose chiefly by reducing hepatic gluconeogenesis and by increasing peripheral insulin sensitivity, without stimulating insulin secretion, so it does not itself cause hypoglycaemia.

Prescribing in practice

• The rare but serious risk is lactic acidosis, so it is contraindicated or must be withheld in significant renal impairment, tissue hypoxia, acute dehydrating illness, and around iodinated contrast and major surgery.
• Gastrointestinal upset is common; introducing it gradually with food or using a modified-release preparation improves tolerability.
• Long-term use can reduce vitamin B12 absorption, which should be considered in patients with anaemia or neuropathy.

Monitoring

Monitor HbA1c and renal function (eGFR), with periodic vitamin B12 checks during prolonged therapy.

Counselling the patient

• Take with or just after meals to reduce nausea and diarrhoea.
• Stop temporarily and seek advice if you become acutely unwell with vomiting, diarrhoea or dehydration, or before a scan using contrast dye.
• It does not usually cause low blood sugar on its own, but this can occur if combined with other diabetes medicines.

Evidence & guidelines

Metformin is the established first-line agent in NICE type 2 diabetes guidance, with long-term cardiovascular benefit demonstrated in the UKPDS.