Metformin ist das meistverordnete Diabetes-Medikament der Welt und seit über 60 Jahren der Goldstandard in der Behandlung von Typ-2-Diabetes. In Deutschland nehmen mehrere Millionen Menschen täglich Metformin ein – es ist das Mittel der ersten Wahl, wenn Ernährungsumstellung und Bewegung den Blutzucker nicht ausreichend senken.
Trotz seiner Wirksamkeit bricht jeder zehnte Patient die Therapie wegen Magen-Darm-Beschwerden ab. In diesem Ratgeber erfährst du, wie Metformin wirkt, wie du die häufigsten Nebenwirkungen vermeidest und was exklusive Daten von brite-Anwendern über die reale Verträglichkeit zeigen.
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Metformin is a prescription medication. Changes to the dose should only be made in consultation with your doctor. This article does not replace medical or diabetic specialist advice.
Metformin is the most widely prescribed antidiabetic medication worldwide and has been in clinical use for over 60 years. It is the cornerstone of type 2 diabetes treatment: inexpensive, extensively studied, weight-neutral, and without hypoglycaemia risk as monotherapy. It has one important pharmacological feature: it is not metabolised in the liver but excreted unchanged by the kidneys — which becomes clinically relevant in renal impairment.
| Property | Details |
|---|---|
| Active substance | Metformin hydrochloride |
| ATC code | A10BA02 (biguanides) |
| Drug class | Oral antidiabetic (biguanide) |
| Available forms | Film-coated tablets (500 mg, 850 mg, 1,000 mg), modified-release tablets, oral solution |
| Half-life | approx. 6.5 hours (no hepatic metabolism) |
| Max. daily dose | 3,000 mg (adults), 2,000 mg (children from age 10) |
| Onset of action | Blood glucose lowering within a few days; full HbA1c effect after 2–3 months |
| Prescription only | Yes |
| Special feature | Weight-neutral to mildly weight-reducing; no hypoglycaemia risk as monotherapy |
Metformin acts on several levels simultaneously — and its mechanism explains both its strengths and its most common side effects. Crucially for patients: metformin does not directly stimulate insulin secretion. It makes the body more efficient in using the insulin already present. This is why metformin as monotherapy does not cause hypoglycaemia.
Metformin's most important and strongest effect occurs in the liver: it inhibits gluconeogenesis — the liver's own production of glucose from amino acids and other precursors. Particularly overnight and in the early morning hours, the liver in people with type 2 diabetes produces too much glucose and releases it into the blood — this explains the often elevated fasting blood glucose in the morning. Metformin specifically suppresses this mechanism. This is why the strongest metformin effect is seen on fasting blood glucose and morning readings.
Metformin improves muscle cells' sensitivity to insulin. In type 2 diabetes, muscle cells are often insulin-resistant — they respond poorly to insulin's signal to take up more glucose. Metformin makes muscle cells more responsive, so that more glucose from the blood can flow into the energy-producing muscles.
Metformin increases glucose uptake in the gut and alters the composition of the gut microbiome. This gut action is probably the main reason for the common gastrointestinal complaints at the start of therapy. Modified-release tablets release the active substance more slowly and place less burden on the gut — which is why they are often better tolerated. Additionally, metformin mildly lowers LDL cholesterol and triglycerides, and may have protective effects against certain cancers currently under investigation in trials.
The most common mistake when starting metformin: beginning with too high a dose too quickly. The majority of gastrointestinal complaints can be avoided by a slow titration over several weeks. This is not an optional recommendation but a pharmacologically justified approach enshrined in all guidelines.
| eGFR (ml/min) | Maximum daily dose | Note |
|---|---|---|
| ≥ 60 | 3,000 mg | No restriction |
| 45–59 | 2,000 mg | Starting dose max. 500–1,000 mg |
| 30–44 | 1,000 mg | Check kidney function every 3–6 months |
| <30 | Contraindicated | Metformin must be stopped |
The correct intake technique is just as important with metformin as the correct dose. The most common complaints — diarrhoea, nausea, bloating — arise almost always from taking it on an empty stomach or from too rapid a dose increase. Both are easily avoided.
Record your intake schedule and all medications in your digital medication plan.
Approximately 15–20% of patients experience gastrointestinal complaints at the start of therapy, and around 5–10% stop therapy for this reason. This is avoidable: almost all these complaints occur when metformin is taken on an empty stomach or at too high a starting dose. With the slow titration schedule from chapter 3 and intake with meals, most complaints resolve on their own within 2–4 weeks.
| Symptom | Frequency | Course |
|---|---|---|
| Diarrhoea | Very common | Usually in the first 2–4 weeks, then improves |
| Nausea | Very common | Often only at the start of therapy |
| Abdominal pain / bloating | Very common | Improves when taken with meals |
| Loss of appetite | Very common | May contribute to weight loss |
| Metallic taste | Common | Usually resolves on its own |
This is the side effect most frequently overlooked. Metformin inhibits vitamin B12 absorption in the gut — via a mechanism involving the intrinsic factor-calcium complex. Between 10 and 30% of long-term users develop a clinically relevant vitamin B12 deficiency over time.
The insidious problem: the symptoms of vitamin B12 deficiency — tingling and numbness in the hands and feet, fatigue, difficulty concentrating — are identical to those of diabetic neuropathy. Many patients (and doctors) automatically attribute these complaints to the diabetes, without considering metformin as a possible cause. Yet the solution is simple: a vitamin B12 blood test and supplementation if needed.
Particularly relevant: anyone also taking pantoprazole or another PPI faces a doubled risk — PPIs also inhibit vitamin B12 absorption. The combination metformin + PPI is extremely common in practice and should always be accompanied by regular B12 monitoring.
Lactic acidosis is the most feared metformin complication — and at the same time the rarest: fewer than 1 case per 10,000 patient-years. It occurs when lactic acid accumulates in the blood because the body cannot break it down quickly enough. This occurs almost exclusively when metformin is taken under circumstances that are actually contraindications — severe kidney failure, dehydration, heart failure with oxygen deficiency, or after contrast dye administration.
Metformin has a manageable interaction profile — but the few clinically relevant interactions are significant. The two most important: alcohol and iodinated contrast dye. Check all combinations with the interaction check.
This interaction is highly clinically relevant and frequently forgotten. Iodinated contrast dye can acutely impair kidney function — sometimes even in patients with previously normal renal values. Since metformin is renally excreted, a sudden decline in kidney function can lead to a rise in metformin levels and thereby to lactic acidosis risk. The rule: stop metformin 48 hours before CT or angiography, check kidney values after the procedure, and only then restart. This information must be actively communicated at every CT registration.
| Substance / medication | Interaction | Recommendation |
|---|---|---|
| Alcohol | Increases lactic acidosis risk; can enhance hypoglycaemia | Only in small amounts, never on an empty stomach. With chronic use: metformin contraindicated |
| Iodinated contrast dye (CT) | Acute renal impairment → metformin accumulation | Stop 48h before and after CT; only restart after renal check |
| NSAIDs (ibuprofen, diclofenac) | Can impair kidney function | Short-term use usually possible; monitor kidneys with long-term use. Prefer paracetamol (acetaminophen) |
| ACE inhibitors / sartans | Can affect kidney function; also blood glucose-lowering | Monitor kidney values; dose adjustment may be needed |
| Insulin / sulphonylureas | Combination increases hypoglycaemia risk | Monitor blood glucose more frequently; adjust insulin dose if needed |
| Corticosteroids (prednisolone) | Raises blood glucose; may reduce metformin effect | Close blood glucose monitoring |
| Diuretics (furosemide, torasemide) | Can lead to dehydration and renal impairment | Drink adequate fluids; monitor kidney values |
