What Is Metformin?

Metformin (brand names: Glucophage, Glumetza) lowers blood sugar by reducing glucose production in the liver and improving insulin sensitivity. It is almost always the first medication prescribed for type 2 diabetes.

How it lowers blood sugar

Metformin works in three ways: (1) reduces liver glucose output by ~30%, (2) increases muscle glucose uptake, (3) slows intestinal glucose absorption. It does NOT cause hypoglycaemia on its own.

Standard Dosing Schedule

Phase	Dose	Timing
Week 1–2	500 mg	Once daily with evening meal
Week 3–4	500 mg	Twice daily (morning + evening)
Week 5+	1000 mg	Twice daily (target dose)
Maximum	2550 mg	Split across 3 meals

Blood Tests to Monitor on Metformin

Test	Frequency	Why
HbA1c	Every 3–6 months	Check diabetes control
eGFR (kidney function)	Every 6–12 months	Metformin is stopped if eGFR <30
Vitamin B12	Annually	Metformin reduces B12 absorption
Liver function	Annually	Baseline + monitoring
FBC	Annually	Check for macrocytic anaemia from B12 deficiency

The B12 Problem Nobody Warns You About

Metformin interferes with how the last part of your small intestine takes up vitamin B12. The absorption step depends on calcium, and metformin disrupts it, so B12 crosses the gut wall less efficiently for as long as you keep taking the drug.² Your liver holds a large reserve, which is exactly why this is easy to miss: the stores absorb the shortfall for years before anything shows in your blood.

The long-term data are clear about the size of the effect. In the Diabetes Prevention Program Outcomes Study, participants were followed for over a decade on metformin or placebo. At five years, low B12 was roughly twice as common on metformin (4.3% against 2.3%), and low or borderline-low B12 affected 19.1% of the metformin group against 9.5% on placebo. Each additional year of metformin raised the odds of deficiency by about 13%.¹

Why this matters more in diabetes than anywhere elseB12 deficiency causes tingling, numbness and burning in the feet — the same picture as diabetic nerve damage. It is easy for both patient and prescriber to file those symptoms under the diabetes and never test for the vitamin. In the same study, nerve problems were more common in metformin users who had low B12.¹ The distinction is worth making, because one of these causes is treatable and the other largely is not.

The practical response is simple: have B12 checked periodically, and particularly once you have been taking metformin for several years, when liver stores are likely to be running down.² A few things speed that depletion — long-term acid-suppressing medicines, previous bariatric surgery, and older age. If your B12 sits at the borderline where the result is hard to interpret, methylmalonic acid and homocysteine can identify a true deficiency earlier than the B12 level alone.²

None of this is a reason to stop metformin, which remains a first-line drug for good reasons. It is a reason to add one test to your monitoring and to say something if your feet start to tingle, rather than assuming it is inevitable.

Why the Stomach Side Effects Usually Pass

Most people starting metformin get some diarrhoea, nausea or cramping, and most find it settles over a few weeks. This is why doses are started low and stepped up slowly rather than begun at the target dose: the titration exists to give your gut time to adapt, not to check whether the drug is working.

If symptoms persist, three things help before anyone concludes the drug is intolerable. Taking it with food rather than on an empty stomach reduces the effect substantially. Going back to the previous dose for a few weeks and then climbing more gradually often succeeds where a rapid increase failed. And a modified-release form releases the drug more slowly along the gut, which many people tolerate when standard tablets defeat them. Say so before abandoning the drug, because the alternative is usually a medicine with more to answer for.

Common Side Effects

Nausea, stomach upset (most common, usually improves after 2–4 weeks)
Diarrhoea especially when starting
Metallic taste
Loss of appetite
Vitamin B12 deficiency with long-term use

Stop Metformin and seek urgent care if you haveSevere muscle pain, weakness, difficulty breathing, or abdominal pain. These can be signs of lactic acidosis, a rare but serious complication.

When Metformin Must Be Stopped

Situation	Reason
eGFR <30 mL/min	Risk of lactic acidosis
Contrast dye (CT scan)	Withhold 48 hours before and after
Surgery	Withhold day before and day of
Acute illness / dehydration	Increases lactic acidosis risk
Excessive alcohol	Alcohol + metformin raises lactic acid risk

B12 TipIf you've been on metformin for more than 1 year, ask your doctor to check your Vitamin B12 level. Deficiency is common and easily treated with supplements.

Metformin and Weight Loss

Metformin often causes modest weight loss (1–3 kg) as a side effect, mainly due to reduced appetite and lower calorie absorption. It is not a weight-loss drug but this side effect is considered beneficial in overweight patients with type 2 diabetes.

Can I drink alcohol on metformin?

Light-to-moderate drinking is generally safe, but heavy drinking significantly increases the risk of lactic acidosis. Your doctor may advise avoiding alcohol entirely.

Does metformin cause low blood sugar?

On its own, metformin does not cause hypoglycaemia. Low blood sugar can occur if combined with insulin or sulfonylureas like glipizide.

Can I take metformin if my kidneys are not perfect?

Metformin is safe when eGFR is 45–89. Dose is halved at eGFR 30–44, and stopped completely below eGFR 30.

Does metformin affect the heart?

Yes. Favourably. Metformin has been shown to reduce heart attack risk and cardiovascular mortality, independently of its blood sugar effects.

Why does metformin cause stomach problems?

Metformin irritates the gut lining. Taking it with food, starting at a low dose, and using the extended-release (XR) form greatly reduces GI side effects.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754–61. PMID 26900641 · doi:10.1210/jc.2015-3754
Bell DSH. Metformin-induced vitamin B12 deficiency can cause or worsen distal symmetrical, autonomic and cardiac neuropathy in the patient with diabetes. Diabetes Obes Metab. 2022;24(8):1423–8. PMID 35491956 · doi:10.1111/dom.14734

Metformin: Complete Patient Guide