Metformin (Gestational Diabetes / PCOS)

How it works

It reduces hepatic gluconeogenesis and improves peripheral insulin sensitivity, lowering circulating glucose and insulin without directly stimulating insulin secretion.

Prescribing in practice

• Metformin crosses the placenta but is widely used in pregnancy and lactation; it does not cause maternal hypoglycaemia when used alone, though insulin may still be required if glycaemic targets are not met.
• Withhold around procedures involving iodinated contrast and during acute intercurrent illness, dehydration or significant renal impairment because of the risk of lactic acidosis.
• Gastrointestinal upset is common on initiation; gradual dose titration and taking it with food improve tolerability.

Monitoring

Monitor capillary blood glucose against pregnancy targets, renal function, and in PCOS the relevant metabolic and ovulatory response.

Counselling the patient

• Take with or just after meals to reduce nausea, bloating and diarrhoea.
• It improves blood sugar control but does not replace dietary and lifestyle measures.
• Report persistent vomiting, dehydration or unusual muscle pain and stop temporarily if seriously unwell.

Evidence & guidelines

NICE guidance supports metformin (alone or with insulin) as a first-line option for gestational diabetes, and the MiG trial demonstrated comparable perinatal outcomes to insulin.