Metformin Renal Dosing: What You Need to Know About Kidney Safety and Dosing

When you take metformin, a first-line medication for type 2 diabetes that helps lower blood sugar by reducing liver glucose production and improving insulin sensitivity. It's one of the most prescribed drugs in the world, but it's not safe for everyone — especially if your kidneys aren't working well. Your kidneys filter metformin out of your body. If they’re weak, the drug can build up and cause a rare but serious condition called lactic acidosis. That’s why renal dosing, the adjustment of medication dosage based on kidney function isn’t optional — it’s life-saving.

Doctors check your eGFR, estimated glomerular filtration rate, a blood test that measures how well your kidneys are filtering waste before starting metformin and at least once a year after. If your eGFR is below 30, you shouldn’t take metformin at all. Between 30 and 45, you may be allowed a reduced dose, and above 45, most people can stay on standard doses. But it’s not just about the number — it’s about how fast your kidney function is dropping. A sudden drop in eGFR, even if still above 45, means you need a quick review. Many patients don’t realize their kidneys are declining until it’s too late, especially if they’re older, have high blood pressure, or have had diabetes for years.

Some people think switching to extended-release metformin makes it safer for kidneys, but that’s not true. The extended-release version just releases the drug slower to reduce stomach upset — it doesn’t change how your kidneys handle it. What does help? Regular blood tests, staying hydrated, avoiding NSAIDs like ibuprofen if you can, and telling your doctor if you’re getting ready for any imaging tests that use contrast dye. Those dyes can temporarily wreck kidney function and trigger dangerous metformin buildup.

It’s also common for older adults to keep taking metformin long after their kidneys have slowed down, simply because no one ever rechecked their numbers. A 2021 study found that nearly 1 in 5 seniors on metformin had eGFR below 30 and were still taking it. That’s not negligence — it’s a system gap. You have to be your own advocate. Ask for your eGFR number at every visit. If your doctor doesn’t mention it, ask: "Is my kidney function still okay for metformin?"

The posts below cover everything you need to connect the dots: how metformin affects your body, why kidney health matters more than you think, how dosing changes with age and other conditions, and what alternatives exist if you can’t take it anymore. You’ll find real-world advice from patients who’ve adjusted their meds, doctors who’ve seen the risks up close, and clear guidelines on when to act — not wait.