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Metformin is the most commonly prescribed diabetes drug in the world and has been the gold standard in the treatment of type 2 diabetes for over 60 years. In Germany, several million people take metformin every day — it is the drug of first choice when diet changes and exercise do not sufficiently reduce blood sugar.Despite its effectiveness, every tenth patient discontinues therapy due to gastrointestinal problems. In this guide, you'll learn how metformin works, how to avoid the most common side effects and what exclusive data from BRITE users shows about real tolerability.
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Metformin is prescription-only. Changes to the dosing only in consultation with the doctor. This article does not replace medical or diabetological advice.
Metformin is the most prescribed antidiabetic worldwide and has been in clinical use for over 60 years. It is the foundation of type 2 diabetes therapy: cheap, well studied, weight-neutral, and without a hypoglycaemia risk as monotherapy. It has one important pharmacological peculiarity: it is not broken down in the liver but excreted unchanged via the kidneys – which becomes clinically relevant with kidney function disorders.
| Property | Details |
|---|---|
| Active substance | Metformin hydrochloride |
| ATC code | A10BA02 (biguanides) |
| Substance class | Oral antidiabetic (biguanide) |
| Available forms | Film-coated tablets (500 mg, 850 mg, 1,000 mg), modified-release tablets, oral solution |
| Half-life | About 6.5 hours (no metabolism in the liver) |
| Max. daily dose | 3,000 mg (adults), 2,000 mg (children from age 10) |
| Onset of action | Blood sugar lowering within a few days, full HbA1c effect after 2–3 months |
| Prescription status | Yes – prescription-only |
| Special feature | Weight-neutral to slightly weight-reducing, no hypoglycaemia risk as monotherapy |
Metformin acts on several levels at the same time – and its mechanism of action explains both the strengths and the most common side effects. Decisive for patients: metformin does not stimulate insulin release directly. It makes the body more efficient at handling the insulin already present. That is the reason why metformin as monotherapy does not cause low blood sugar.
The most important and strongest effect of metformin takes place in the liver: it inhibits gluconeogenesis – the body's own production of sugar from amino acids and other precursors. Especially at night and in the early morning hours, the liver in type 2 diabetics produces too much glucose and releases it into the blood – this explains the often raised fasting blood sugar in the morning. Metformin suppresses exactly this mechanism. That is why the strongest metformin effect is seen in the fasting blood sugar and in the morning reading.
Metformin improves the sensitivity of the muscle cells to insulin. In type 2 diabetes the muscle cells are often insulin-resistant – they "listen" poorly to insulin's signal to take up more glucose. Metformin makes the muscle cells receptive again, so that more glucose can flow from the blood into the energy-producing muscles.
Metformin increases the glucose uptake in the gut and changes the composition of the gut flora. This gut effect is probably the main reason for the frequent gastrointestinal complaints at the start of therapy. Modified-release tablets release the active substance more slowly and burden the gut less – which is why they are often better tolerated. In addition, metformin slightly lowers LDL cholesterol and triglycerides and possibly has protective effects against certain cancers, which are currently being investigated in studies.
The most common mistake when starting metformin: starting too fast with too high a dose. The majority of gastrointestinal complaints can be avoided by slowly building up over several weeks. This is not an optional recommendation but the pharmacologically justified route, anchored in all guidelines.
| GFR (ml/min) | Maximum daily dose | Note |
|---|---|---|
| ≥ 60 | 3,000 mg | No restriction |
| 45–59 | 2,000 mg | Starting dose a maximum of 500–1,000 mg |
| 30–44 | 1,000 mg | Check kidney function every 3–6 months |
| <30 | Contraindicated | Metformin must be stopped |
The right intake technique is just as important with metformin as the right dose. The most common complaints – diarrhoea, nausea, bloating – almost always arise from taking it on an empty stomach or from too fast a dose increase. Both are easy to avoid.
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About 15–20% of patients have gastrointestinal complaints at the start of therapy, and about 5–10% stop the therapy because of them. This is avoidable: almost all of these complaints occur when metformin is taken on an empty stomach or in too high a starting dose. With the slow build-up schedule from chapter 3 and taking it with a meal, most complaints disappear by themselves after 2–4 weeks.
| Symptom | Frequency | Course |
|---|---|---|
| Diarrhoea | Very common | Mostly in the first 2–4 weeks, then improves |
| Nausea | Very common | Often only at the start of therapy |
| Abdominal pain / bloating | Very common | Improve when taken with a meal |
| Loss of appetite | Very common | Can contribute to weight reduction |
| Metallic taste | Common | Mostly disappears by itself |
This is the side effect that is most frequently overlooked. Metformin inhibits the absorption of vitamin B12 in the gut – via a mechanism that acts on the so-called intrinsic factor–calcium complex. Between 10 and 30% of long-term users develop a clinically relevant vitamin B12 deficiency over time.
The treacherous problem: the symptoms of a vitamin B12 deficiency – tingling and numbness in the hands and feet, fatigue, concentration problems – resemble exactly the symptoms of a diabetic neuropathy. Many patients (and doctors) automatically attribute these complaints to the diabetes, without thinking of metformin as a possible cause. Yet the solution would be simple: a vitamin B12 check and, where appropriate, supplementation.
Particularly relevant: anyone who at the same time takes pantoprazole or another PPI has a double risk – PPIs also inhibit vitamin B12 absorption. The combination of metformin + PPI is extremely common in practice and should always be accompanied by a regular B12 check.
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 arises when lactic acid (lactate) accumulates in the blood because the body cannot break it down fast enough. This happens almost exclusively when metformin is taken under circumstances that are actually contraindications – severe kidney weakness, dehydration, heart failure with oxygen deficiency, or after the administration of contrast medium.
Metformin has a manageable interaction profile – but the few clinically relevant interactions are significant. The two most important: alcohol and iodine-containing contrast media. Check all combinations in the interaction check.
This interaction is highly clinically relevant and is frequently forgotten. Iodine-containing contrast media can acutely worsen kidney function – sometimes even in patients with previously normal kidney values. Since metformin is excreted via the kidneys, a sudden kidney function disorder can lead to a rise in the metformin level and thereby to the lactic acidosis risk. The rule: stop metformin 48 hours before the CT or the angiography, check the kidney values after the procedure, and only then restart. This information must be actively communicated at every CT booking.
| Substance / medication | Interaction | Recommendation |
|---|---|---|
| Alcohol | Increases the risk of lactic acidosis, can enhance low blood sugar | Only in small amounts, never on an empty stomach. With chronic consumption: metformin contraindicated |
| Iodine-containing contrast media (CT) | Acute worsening of kidney function → metformin accumulation | Stop 48 h before and after CT, only restart after a kidney check |
| NSAIDs (ibuprofen, diclofenac) | Can worsen kidney function | Possible short-term, monitor kidney values with long-term intake. Prefer paracetamol |
| ACE inhibitors / sartans | Can affect kidney function, simultaneously blood-sugar-lowering | Check kidney values, dose adjustment if needed |
| Insulin / sulfonylureas | The combination increases the hypoglycaemia risk | Check blood sugar more often, adjust insulin dose if needed |
| Cortisone (prednisolone) | Raises blood sugar, can weaken the metformin effect | Check blood sugar closely |
| Diuretics (furosemide, torasemide) | Can lead to dehydration and worsening of kidney function | Drink enough, check kidney values |
