Researchers say metformin could be reducing vitamin B12 levels in some people with diabetes.

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People with diabetes who take metformin are urged to get their vitamin B12 levels checked. Getty Images

The number one most prescribed drug to treat people with type 2 diabetes may actually be causing irreversible nerve damage in many patients.

Metformin, also known as Glucophage, is prescribed to more than 120 million patients worldwide. It works by reducing the amount of glucose produced by the liver as well as increasing a patient’s sensitivity to insulin.

A recent study from research conducted by two hospitals in the United Kingdom confirms a concern the medical world has been aware of for quite a few years.

Metformin is causing varying degrees of vitamin B12 deficiency in approximately 10 percent of prescribed patients.

The research was done by Dr. Kaenat Mulla and colleagues at Hucknall Road Medical Centre in Nottingham.

The researchers conducted an audit of vitamin B12 screening and deficiency among female patients with type 2 diabetes who were taking metformin.

“The audit findings indicated that 64 percent of patients had not had their vitamin B12 levels checked at all, and that 9.6 percent of patients were deficient but only 6.4 percent were being treated with vitamin B12,” a press release from the study states.

Why B12 is important

Vitamin B12 plays a critical role in the development and function of brain and nerve function.

The most significant concern is that even a patient who is managing their blood sugars at healthy levels could still experience severe and permanent nerve damage.

That’s because vitamin B12 is essential to the formation and maintenance of the myelin sheath — a fatty substance that protects your central and peripheral nervous system.

For people with diabetes who take metformin, that permanent nerve damage could easily be incorrectly diagnosed as peripheral neuropathy — a known complication of high blood sugars in people with diabetes.

“Nerve damage in the periphery (e.g. face, limbs, organs),” explained the press release, “is a common complication of diabetes, with symptoms that range from numbness to pain, and can lead to debilitating loss of balance and coordination.”

A potentially dangerous diagnosis

That diagnosis of peripheral neuropathy could be falsely attributed to the patient’s blood sugar management rather than to the side effects of the medication being used to treat the patient’s diabetes.

“Current British Society of Haematology guidelines recommend that vitamin B12 levels are checked only when there is clinical suspicion of deficiency,” explained Mulla. “However, peripheral neuropathy is irreversible and it may be too late once symptoms have developed.”

An unknowing doctor might even prescribe a higher dose of metformin to improve the patient’s blood sugar levels, not realizing that indeed a side effect of the drug itself is the culprit.

“I’ve been hearing about this since about five years ago,” Dr. Caroline Messer, an endocrinologist and clinical assistant professor at Mount Sinai School of Medicine, told Healthline. “But there are no guidelines established for testing for it.”

Messer said the problem is that while some doctors are testing their patients’ vitamin B12 levels, many doctors are not because it hasn’t become an official part of standardized diabetes care.

“In my office, patients on metformin who are B12 deficient isn’t something we see often. It’s rare here, but this study says 10 percent of all patients taking metformin are deficient, and that’s not a small amount,” she said.

What B12 deficiency does

Messer emphasizes that once the nerve damage is done, it cannot be fixed.

However, you can prevent further nerve damage after a B12 deficiency is identified through a basic blood test by taking supplemental B12.

Symptoms of B12 deficiency include diarrhea (which is also the most common side effect of metformin), constipation, exhaustion, loss of appetite, pale skin, inflamed red tongue, and bleeding gums.

The consequences of a B12 deficiency include permanent nerve damage, deteriorating brain function, memory loss, and temporary infertility in women.

In people without diabetes, B12 deficiency is considered common with as much as 15 percent of the American population deficient, according to the National Institutes of Health.

While it’s possible that metformin simply causes malabsorption of B12, Messer says there is no conclusive theory to explain why patients taking the drug become deficient.

And it isn’t just in patients who otherwise tolerate the drug well compared to patients who experience diarrhea and other gastric distress side effects.

Should you take a supplement?

Messer says taking a supplement can treat the deficiency and prevent further nerve damage while still taking metformin as prescribed.

A normal B12 level is considered anywhere between 190 and 900 nanograms per milliliter (ng/mL), but Messer suggests treating any level below 300 ng/mL.

“Just have your levels checked every year if you take metformin,” she said. “For patients with levels over 300 mg/mL, taking a supplement isn’t necessary.”

Mulla added the same caution, concluding that patients shouldn’t stop taking metformin because of this research.

“Metformin remains the best treatment for type 2 diabetes. These findings should not discourage patients from taking it, but encourage doctors to monitor vitamin B12 levels more routinely, so any deficiency can quickly be treated.”

Ginger Vieira is an expert patient living with type 1 diabetes, celiac disease, and fibromyalgia. Find her diabetes books on Amazon and connect with her on Twitter and YouTube.