It's quite common to visit the doctor because your toes feel numb and hear, "It's because you've had diabetes for a long time." That statement might be true.

However, if you've been taking metformin for several years, make sure to ask one more question before leaving the doctor's office: "When was the last time my vitamin B12 levels were checked?"

First, let me clarify: I'm not suggesting you stop taking the medication.

Metformin is still widely used for diabetes, and I'm not writing this to imply that the drug is bad. It's just that the longer you take it, the more tests you need to keep track of.

The reason lies in absorption. According to NIH data, metformin can interfere with the process of B12 absorption in the intestines.

This information is also included as a Warning on the FDA-approved label. It states that in a 29-week clinical trial, 7% of patients with previously normal B12 levels experienced a drop below normal.

Only 7% in 29 weeks surprised me. What about those who have been taking it for several years or even over a decade?

A study from the long-term follow-up of the American Diabetes Prevention Program provides a closer answer. They divided high-risk adults into metformin and placebo groups and measured B12 levels at 5 and 13 years.

At 5 years, the percentage of people with low or borderline B12 levels (298 pg/mL or lower) was 19.1% in the metformin group and 9.5% in the placebo group. At 13 years, it was 20.3% versus 15.6%.

There was also an analysis showing that for each additional year of use, the odds of deficiency increased by 1.13 times. This indicates that the longer you take it, the more attention you need to pay.

To be fair, there are limitations. When looking only at the group with 'definitely low' levels (203 pg/mL or lower), the difference at 13 years (7.4% vs. 5.4%) was not statistically significant.

Still, the researchers' conclusion is clear: routine B12 testing should be considered for metformin users.

The tricky part is the symptoms. Symptoms identified by NHLBI and MedlinePlus include fatigue, pale skin, numbness in hands and feet, balance issues, concentration and memory problems, and a red, painful tongue.

Does that sound familiar? Numbness in the hands and feet overlaps with diabetic neuropathy, and fatigue and forgetfulness can easily be attributed to aging.

The UK's Medicines and Healthcare products Regulatory Agency (MHRA) also points out that these symptoms could be due to diabetes or other conditions. So, just looking at the symptoms won't help distinguish the cause; a blood test is necessary.

Why is it a problem if you wait too long? Anemia tends to respond well to treatment, but nerve damage can become permanent if treatment is delayed.

NHLBI also advises that neurological symptoms like numbness may not resolve even with treatment. Missing out on something that can be easily checked with a simple blood test would be very unfortunate.

So, how often should you be tested? This is where recommendations vary a bit.

The FDA label suggests checking blood levels annually, and B12 every 2-3 years. The American Diabetes Association (ADA) recommends regular checks for those with anemia or peripheral neuropathy, and suggests annual testing for those who have been taking it for over four years or have risk factors.

If it were me, I would ask to include a B12 test during my annual blood work if I've been on it for a long time. Ideally, the healthcare system should take care of this before the patient has to, but unfortunately, that's not always the case.

There are also specific groups that need to be more vigilant. MHRA identifies risk factors such as older age, high-dose use, gastrointestinal surgeries or conditions like Crohn's disease, and strict vegetarianism.

If you're also taking a proton pump inhibitor (PPI), which reduces stomach acid, make sure to mention that if you've been on reflux medication for a long time.

So, what should you do? Let's summarize the steps. First, check your metformin usage duration and the date of your last B12 test at your next appointment.

Second, if you experience numbness, fatigue, or forgetfulness, directly ask, "Can I get a B12 test?" Also, request a CBC blood test.

Third, don't just feel relieved by the numbers on the results. NHLBI states that even if B12 levels are normal, you could still be deficient, and if it's ambiguous, your doctor may consider additional tests like MMA (methylmalonic acid).

Fourth, even if you test deficient, do not stop taking metformin on your own. MHRA advises against discontinuing without consulting a doctor, and you can continue taking it while supplementing B12.

Treatment can involve pills, injections, or nasal sprays, depending on the cause and severity. The recommended daily intake for adults is 2.4 mcg, and those over 50 may need fortified foods or supplements due to decreased absorption, according to NIH recommendations.

If you take a lot of folic acid supplements, remember this: high doses of folic acid can mask anemia caused by B12 deficiency, delaying the recognition of nerve damage.

Make sure to bring a list of all medications and supplements to discuss with your doctor or pharmacist, as the answers can vary based on individual circumstances.

November is American Diabetes Month. If you've been on metformin for a long time, take a moment this month to check one more item on your blood test list.