Metformin vs Gliclazide
Clinical Comparison
Clinical Context
Metformin is universally first-line for type 2 diabetes. Gliclazide is the sulfonylurea of choice in the UK, used when metformin alone does not achieve HbA1c targets, or when metformin is contraindicated (e.g. severe renal impairment). NICE NG28 positions sulfonylureas as one of several second-line options alongside SGLT2 inhibitors, DPP-4 inhibitors, and pioglitazone.
Drug Profiles
Metformin
Biguanide
Mechanism
Activates AMP-activated protein kinase (AMPK), reducing hepatic glucose output, increasing peripheral glucose uptake, and improving insulin sensitivity
Indications
- Type 2 diabetes (first-line)
- Polycystic ovary syndrome (off-label)
- Pre-diabetes / diabetes prevention
Common Doses
500 mg BD initially, titrated to 1 g BD (max 2 g/day; MR preparation: up to 2 g OD)
Route
Oral
Onset & Duration
Onset within days; maximal glycaemic effect 2-4 weeks
Gliclazide
Sulfonylurea
Mechanism
Binds to SUR1 receptors on pancreatic beta cells, blocking KATP channels, causing depolarisation and insulin release independent of glucose levels
Indications
- Type 2 diabetes (second-line after metformin, or first-line if metformin contraindicated)
Common Doses
40-80 mg OD initially, up to 160 mg BD (MR: 30-120 mg OD)
Route
Oral
Onset & Duration
Onset 1-2 hours; duration 12-24 hours (MR formulation)
Key Differences
| Category | Metformin | Gliclazide |
|---|---|---|
| Position in guidelines | First-line for all type 2 diabetes (NICE NG28) | Second-line or first-line only if metformin contraindicated |
| Hypoglycaemia risk | No hypoglycaemia as monotherapy | Significant hypoglycaemia risk |
| Weight effect | Weight neutral / slight loss | Weight gain (2-3 kg typical) |
| Cardiovascular benefit | Proven CV benefit (UKPDS) | No proven CV benefit |
| Renal impairment | Reduce dose eGFR 30-45; stop <30 | Safer in CKD (hepatic metabolism) but hypoglycaemia risk increases |
| Mechanism | Insulin sensitiser — does not increase insulin secretion | Insulin secretagogue — stimulates beta cell insulin release |
Position in guidelines
First-line for all type 2 diabetes (NICE NG28)
Second-line or first-line only if metformin contraindicated
Hypoglycaemia risk
No hypoglycaemia as monotherapy
Significant hypoglycaemia risk
Weight effect
Weight neutral / slight loss
Weight gain (2-3 kg typical)
Cardiovascular benefit
Proven CV benefit (UKPDS)
No proven CV benefit
Renal impairment
Reduce dose eGFR 30-45; stop <30
Safer in CKD (hepatic metabolism) but hypoglycaemia risk increases
Mechanism
Insulin sensitiser — does not increase insulin secretion
Insulin secretagogue — stimulates beta cell insulin release
Key Advantages
Metformin
- Weight neutral or promotes modest weight loss
- No hypoglycaemia risk as monotherapy
- Cardiovascular benefit (UKPDS)
- First-line per NICE NG28 for all type 2 diabetes
- Very inexpensive
Gliclazide
- Potent glucose lowering (HbA1c reduction ~1-1.5%)
- Fast onset — useful when rapid control needed
- Lower hypoglycaemia risk than glibenclamide (shorter-acting)
- Preferred sulfonylurea in renal impairment (hepatic metabolism)
Key Cautions
Metformin
- GI side effects (nausea, diarrhoea) — start low, increase slowly
- Lactic acidosis risk if eGFR <30 (contraindicated)
- Withhold before iodinated contrast and for 48 hours after
- Reduce dose if eGFR 30-45; stop if <30
- B12 deficiency with long-term use
Gliclazide
- Hypoglycaemia risk (especially in elderly, renal impairment, missed meals)
- Weight gain (typically 2-3 kg)
- Requires functioning beta cells — ineffective in type 1 diabetes
- Caution in hepatic impairment
- Secondary failure over time as beta cell function declines
Clinical Verdict
Metformin is always first-line due to no hypoglycaemia risk, weight neutrality, and cardiovascular benefit. Add gliclazide as second-line if HbA1c remains above target, but counsel on hypoglycaemia and weight gain.
Medical Disclaimer: This comparison is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare professional before making prescribing decisions. Verify all drug information with current clinical guidelines (BNF, NICE, SmPCs).
Full monographs for Metformin & Gliclazide in the MedNext app
Complete dosing, interactions, contraindications, side effects, and more — covering 2,866 drugs with AI-powered search.
Download MedNext Formulary