How it's calculated
This tool shows the standard adult starting dose and titration schedule, plus the eGFR-based renal restriction band if an eGFR is entered — metformin dosing is not weight-based.
eGFR-based renal restrictions
| eGFR (mL/min/1.73m²) | Guidance |
|---|---|
| ≥ 45 | No renal dose restriction |
| 30-44 | Reduce dose (commonly max 1000 mg/day); do not newly initiate |
| < 30 | Contraindicated — discontinue |
Clinical use
- First-line oral agent for type 2 diabetes in most guidelines, given its efficacy, weight neutrality, low hypoglycaemia risk, and cardiovascular safety data.
- Renal function must be checked before starting and monitored at least annually thereafter (more often in those with, or at risk of, renal impairment), since metformin accumulation in renal impairment increases lactic acidosis risk.
- Temporarily withhold metformin in any acute illness with a risk of dehydration or hypoxia (e.g. severe vomiting/diarrhoea, sepsis, major surgery, acute heart failure) — the classic 'sick day rules'.
Frequently asked questions
Why is metformin dose based on eGFR rather than weight?
Metformin's efficacy doesn't scale meaningfully with body weight the way antibiotic or sedative dosing does — the dose-limiting concern is renal clearance, so dosing is guided by kidney function (eGFR) rather than weight.
Why is metformin contraindicated at eGFR below 30?
Metformin is renally cleared, and significant accumulation in advanced renal impairment substantially raises the risk of lactic acidosis, a rare but serious and potentially fatal adverse effect.
Can metformin be used around contrast imaging?
Current guidance has narrowed blanket contrast-related holds to patients with eGFR below 30 or additional AKI risk factors — check your local radiology/renal protocol, as this has changed over time from the older 'hold for all contrast' rule.
References
- American Diabetes Association. Standards of Care in Diabetes — Pharmacologic Approaches to Glycemic Treatment.
- FDA Drug Safety Communication — Metformin-containing drugs: Drug Safety Communication - Revised Warnings for Certain Patients With Reduced Kidney Function.