Biguanide — Gestational Diabetes / PCOS Pregnancy: Metformin crosses the placenta with levels that can be as high as maternal concentrations. A large amount of data on pregnant women (more than 1,000 exposed outcomes) indicates no increased risk of congenital abnormalities or feto/neonatal toxicity after exposure in the periconceptional phase and/or during pregnancy. If clinically needed, the use of metformin can be considered during pregnancy and in the periconceptional phase as an addition or an alternative to insulin. Breast-feeding is not recommended during metformin treatment (metformin is excreted into human breast milk; only limited data available).

Metformin (Gestational Diabetes / PCOS)

Brand names: Glucophage

Metformin is an oral biguanide antihyperglycaemic used in obstetrics and gynaecology for gestational diabetes and as an insulin-sensitising agent in polycystic ovary syndrome, including to support ovulation and reduce metabolic risk.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Usual starting dose 500 mg or 850 mg metformin hydrochloride, 2 or 3 times daily, given during or after meals; after 10 to 15 days the dose should be adjusted on the basis of blood glucose measurements

Route: Oral

Frequency: Two or three times daily, during or after meals

Max: 3 g metformin hydrochloride daily, taken as 3 divided doses (adults with normal renal function, GFR ≥ 90 mL/min)

INDICATION CAVEAT — READ FIRST: the fetched UK SPC (Metformin 1000 mg film-coated tablets) gives posology for metformin as an oral antidiabetic (type 2 diabetes) only. It contains NO separate regimen for gestational diabetes and NO regimen for polycystic ovary syndrome; use in those settings is outside the SPC posology and the regimen must be confirmed against national/local guidance. The SPC §4.6 does state that 'if clinically needed, the use of metformin can be considered during pregnancy and in the periconceptional phase as an addition or an alternative to insulin'. SPC §4.2 detail: a slow increase of dose may improve gastrointestinal tolerability. If transferring from another oral antidiabetic agent, discontinue the other agent and initiate metformin at the dose above. COMBINATION WITH INSULIN: metformin is given at the usual starting dose of 500 mg or 850 mg 2 or 3 times daily while the insulin dose is adjusted on the basis of blood glucose measurements. ELDERLY: adjust dosage based on renal function; regular assessment of renal function is necessary. PAEDIATRIC (from the SPC, non–per-kg so not structured below): metformin can be used in children from 10 years of age and in adolescents; usual starting dose 500 mg or 850 mg once daily during or after meals, adjusted after 10 to 15 days on the basis of blood glucose; maximum recommended dose 2 g daily in 2 or 3 divided doses. Verify any paediatric dose against a children's formulary. Interactions listed below are drawn from SPC §4.3/§4.4 — §4.5 was not captured in this bundle.

Dose adjustments

Renal

Assess GFR before initiation and at least annually thereafter (every 3–6 months in patients at increased risk of progression and in the elderly). Total maximum daily dose, divided into 2–3 daily doses: GFR 60–89 mL/min — 3,000 mg (dose reduction may be considered with declining renal function); GFR 45–59 mL/min — 2,000 mg (review risk factors for lactic acidosis before initiation; starting dose at most half the maximum dose); GFR 30–44 mL/min — 1,000 mg; GFR < 30 mL/min — metformin is contraindicated. Temporarily discontinue in conditions that alter renal function (e.g. dehydration from severe diarrhoea or vomiting, fever, reduced fluid intake).

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Any type of acute metabolic acidosis (such as lactic acidosis, diabetic ketoacidosis); diabetic pre-coma
Severe renal failure (GFR < 30 mL/min)
Acute conditions with the potential to alter renal function: dehydration, severe infection, shock
Disease which may cause tissue hypoxia (especially acute disease or worsening of chronic disease) such as decompensated heart failure, respiratory failure, recent myocardial infarction, shock
Hepatic insufficiency, acute alcohol intoxication, alcoholism

Side effects

Very common: gastrointestinal disorders — nausea, vomiting, diarrhoea, abdominal pain and loss of appetite (most frequent at initiation; usually resolve spontaneously)
Common: vitamin B12 decrease/deficiency
Common: taste disturbance
Very rare: lactic acidosis (serious — acidotic dyspnoea, abdominal pain, muscle cramps, asthenia, hypothermia followed by coma)
Very rare: liver function test abnormalities or hepatitis, resolving on discontinuation
Very rare: skin reactions such as erythema, pruritus, urticaria

Interactions

Iodinated contrast media — intravascular administration may cause contrast-induced nephropathy with metformin accumulation and increased risk of lactic acidosis; metformin should be discontinued prior to or at the time of the imaging procedure (SPC §4.4)
Medicinal products that can acutely impair renal function (antihypertensives, diuretics, NSAIDs) — initiate with caution in metformin-treated patients (SPC §4.4)
Alcohol — excessive intake and acute alcohol intoxication increase the risk of lactic acidosis (also a contraindication) (SPC §4.3/§4.4)
Other medicinal products that may cause lactic acidosis — concomitant use is a risk factor (SPC §4.4)

Clinical monograph

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.

Reference: NICE NG3 (Diabetes in Pregnancy); MiG Trial (Rowan et al. NEJM 2008); ESHRE/ASRM PCOS Guidelines 2023; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators