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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Biguanides

Oral glucose-lowering drugs that make the liver release less glucose. Metformin is the only one still in wide use.

Updated 2026-11-01

## What they are

Guanidine-derived oral drugs for type 2 diabetes. They lower glucose without raising insulin secretion.

## Mechanism of action

Biguanides accumulate in liver cells and mildly inhibit mitochondrial complex I. The cell's energy charge falls (less ATP, more AMP), AMPK is activated and hepatic gluconeogenesis drops. Which of these effects matters most, and how much the gut contributes, is still debated.

Target: Mitochondrial respiratory chain, complex I (with AMPK activation downstream)

## Key points

- First-line drug for type 2 diabetes in most guidelines, alongside lifestyle changes.
- Does not cause hypoglycemia on its own and is weight-neutral or causes slight weight loss.
- Most feared adverse effect: lactic acidosis, mostly with severe kidney failure, hypoxia, shock or heavy alcohol use.
- Cleared unchanged by the kidney: kidney function decides whether it can be used.
- Long-term use lowers vitamin B12.

## Drugs in this class

- [**Metformin** First-line oral medicine for type 2 diabetes: it mainly lowers the glucose the liver makes, and alone it rarely causes… Mitochondrial respiratory chain, complex I](https://durgical.com/en/pharmacology/metformin/)

## Others in the class

- phenformin (withdrawn in most countries because of lactic acidosis)
- buformin

## Test yourself

Think of the answer, then tap the question to see it.

## Why was phenformin withdrawn while metformin remained in use?

Phenformin caused lactic acidosis far more often. Metformin is more polar, enters mitochondria less avidly and carries a much lower, though real, risk.

## Do biguanides increase insulin secretion?

No. They reduce hepatic glucose output and improve insulin action, which is why they rarely cause hypoglycemia when used alone.

## Which organ function must be checked before and during biguanide treatment, and why?

Kidney function: metformin is excreted unchanged in urine, so a falling filtration rate lets it accumulate and raises the risk of lactic acidosis.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Metformin: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a696005.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A10BA02) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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