Is metformin bad for your kidneys? There’s a lot of misinformation and confusion regarding the answer to this question.  That’s why a few months ago I created a survey to ask physicians about their use of metformin in chronic kidney disease (CKD) patients. The results suggested that many practitioners are following outdated clinical guidelines when prescribing metformin in certain stages of CKD. As a result, their patients may be missing out on the kidney health benefits of metformin. In this blog, I will review the evidence and current recommendations for using metformin in CKD.


Metformin and CKD survey results

You can see the results of the survey in this earlier blog. It demonstrated that caution about the use of metformin in CKD is common among primary care physicians and other specialties. Nephrologists seem to be more comfortable with using it until the patient reaches stage 4 CKD. Most nephrologists, however, were not aware that metformin is associated with nutritional deficiencies. You can read the results of the survey in detail here.

What is metformin?

Metformin was first produced in 1922. However, since 1958 it has been used in the control of blood sugar in type 2 diabetes. It is a biguanide drug, a class of medications that inhibit the liver’s production of glucose, enhancing the body’s insulin sensitivity and lowering the amount of blood sugar that the intestines absorb.  It is chemically developed from galegina, a natural guanidine compound found in Galega officinalis (French lilac, also known as goat’s rue).
In essence, metformin is an oral diabetes medication that helps control blood sugar levels. It is often used together with diet and exercise to improve blood sugar control in patients with type 2 diabetes mellitus. There are other off-label uses for metformin and it is currently being studied for the treatment of polycystic kidney disease.

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Does metformin cause acute kidney injury?

For many years clinicians have been avoiding the use of metformin in kidney patients and specifically in those with diabetes. This is fueled by the FDA warning about the use of metformin in these patients due to concerns about lactic acidosis (discussed later). The original FDA warning was much more strict, but the more recent one issued in 2016 weighed in with more detail about metformin and kidney function. The new warning allows metformin to be used in patients who have worse kidney function. The newest warning indicates metformin can be used in patients who have an estimated glomerular filtration rate (eGFR) greater than 45 mL/min. Furthermore,  metformin can continue to be used in existing patients whose eGFR remains over 30 mL/min.
As you saw from the survey, many primary care physicians and other specialists think that metformin can cause direct acute kidney injury. However, the literature shows that metformin can be protective in kidney disease. There is absolutely no evidence that metformin can lead to kidney injury. In fact, there is evidence to the contrary.

Metformin clearance by the kidneys

Metformin does not undergo metabolism in the body, and it is eliminated unchanged in the urine. It is mainly cleared by tubular secretion in the kidneys. Metformin clearance is proportional to creatinine clearance. Therefore, although metformin can be used in patients with advanced kidney disease the dose of metformin should be decreased. The following table shows the recommended doses of metformin according to kidney function.

Kidney function Total daily metformin dose
Normal 3000 mg
45-59 ml/min 2000 mg
30-44 ml/min 1000 mg
Less than 30 ml/min Not recommended


Metformin-induced diarrhea

The most common side effects of metformin are gastrointestinal. This is because one of the ways that metformin works is directly on the gut, altering glucose (sugar) absorption in the gut. Consequently, symptoms of nausea and diarrhea can occur especially in the first few weeks of taking it. That is why the patient should be started on a low dose to get used to it. Despite the statistic that 25% of patients taking metformin report diarrhea, only about 5% of those taking it had to stop it due to that side effect.

Metformin and lactic acidosis

The real reason why metformin was technically “banned” from use in CKD patients in previous years was due to the development of lactic acidosis. Lactic acidosis is a disease where lactic acid, a metabolic byproduct, accumulates in the blood. Lactic acid is mainly produced in small amounts by the muscles and red blood cells. It is formed when the body breaks down sugars to make energy under conditions of low oxygen (anaerobic glycolysis). This can occur during intense exercise or severe infection (called sepsis).

Metformin can induce lactic acidosis by shifting energy production in the cells toward anaerobic glycolysis. It also decreases the ability of the liver to consume lactic acid. Yet, in practice this is rare. In fact, it has been shown that lactic acidosis can occur in patients taking other oral blood-sugar-lowering medications at the same rate as metformin. Most case reports of metformin-induced lactic acidosis were in patients who have other conditions that lead to acute kidney injury (AKI) and lactic acidosis such as septic shock.

So, it is not that:

Instead, what happens is:


Finally, in a large retrospective study of patients with diabetes mellitus, metformin use was not significantly associated with lactic acidosis at kidney function levels higher than 30 ml/min/1.73 m2 (stage 3 or better).

Metformin and vitamin B12 deficiency

The association between long-term use of metformin and low vitamin B12 levels in type 2 diabetes mellitus patients is well-established. This is thought to be due to changes in the production of intrinsic factor in the stomach, which decreases vitamin B12 absorption in the intestine. Vitamin B12 deficiency is more pronounced if the patient is also taking a proton pump inhibitor such as omeprazole or histamine H2 receptor antagonist such as famotidine, which are medications used to treat acid reflux. Since diabetes can cause peripheral neuropathy, as well as vitamin B12 deficiency, can cause peripheral neuropathy, it can be confusing for diabetic patients on metformin and their practitioners.

Therefore, patients using metformin for a prolonged period should be screened initially and periodically for vitamin B12 deficiency. In addition, there is also growing evidence that other B vitamins, vitamin D, and magnesium can be impacted by metformin.

Bonus: Metformin’s impact on the gut microbiome

Because of metformin’s actions on the gut, it can also alter the intestinal microbiota for the better. Studies showed that metformin can enhance the growth of Akkermansia muciniphila, Butyrivibrio, Bifidobacterium, and Lactobacillus.  In addition, it decreases the levels of some other bacteria like Intestinibacter. Metformin also promotes the production of the short-chain fatty acids that protect the intestinal barrier. In fact, some authors attribute some of metformin’s actions to its favorable impact on the gut microbiota and integrity.

The Bottom Line for Metformin and Kidney Disease

The answer to the question “Is metformin bad for the kidneys?” is generally no. Metformin can be used in patients with kidney disease. It actually leads to better outcomes in these patients. However, the risk of metformin-induced lactic acidosis increases in CKD stage 4 and above. It is, therefore, not recommended for use in these patients and patients with acute kidney injury. I also recommend avoiding it in patients with renovascular disease because their kidney function tends to fluctuate. Furthermore, metformin can be associated with vitamin B12 deficiency and patients should be screened initially and periodically for deficiency of this important vitamin.