Metformin half-life
Metformin is a prescription biguanide, and its plasma half-life is about 6.2 hours, meaning roughly half of a given amount clears the blood in that window. Because it clears fairly quickly, is excreted unchanged by the kidneys, and produces no active metabolites, the immediate-release form is taken twice a day, while the extended-release form uses a slow-release system to cover about 24 hours from one daily dose. A separate measurement in whole blood runs much longer, near 17.6 hours, because the drug collects in red blood cells, but the 6.2 hour plasma value is the one that sets the dosing schedule.
What a half-life actually means
A half-life is the time it takes for half of what is in your blood to be gone. It does not mean the compound is finished at that point. Half goes, then half of what is left goes, and so on.
That is why the last traces take so much longer to disappear than the first. Most references treat something as effectively cleared after about five half-lives, which for Metformin is roughly 30 hours.
How Metformin clears
| Half-lives | Time since dose | Roughly left |
|---|---|---|
| 1 | 6 hours | 50% |
| 2 | 12 hours | 25% |
| 3 | 18 hours | 12.5% |
| 4 | 24 hours | 6.25% |
| 5 | 30 hours | 3.125% |
These are round numbers from a single half-life figure. Real clearance shifts with dose, body composition, liver and kidney function, and what else you are taking. Treat it as a shape, not a schedule.
Why the timing matters
A short half-life means levels swing between doses, so timing and frequency do most of the work. A long half-life means levels build for days or weeks before they settle, so a change you make today might not show up for a while, and stopping does not clear things straight away.
Metformin sits at about 6 hours, so it behaves like the short end of that range.
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Indexed records from PubMed and ClinicalTrials.gov that reference Metformin. These are here so you can read the primary material yourself. They are not the source of the half-life figure above.