Metformin and Vitamin B12: Can You Take Them Together?

Why you are probably reading this

You have PCOS (now renamed PMOS). Your doctor put you on metformin for insulin resistance, irregular cycles, or to help with fertility. Somewhere along the way you started a prenatal or a multivitamin, or you read that metformin "depletes B12," and now you are holding a bottle wondering whether you are supposed to take it, avoid it, or space it out.

Here is the whole picture, in the order you need it.

What metformin actually does to B12

B12 from food or a supplement has to be picked up in the last part of your small intestine (the ileum). To get in, it has to be attached to a protein called intrinsic factor, and the receptor that grabs that complex needs calcium to work.

Metformin interferes with that calcium-dependent step. In a small but well-designed 2000 study, patients switched from a sulfonylurea to metformin showed a steady decline in B12 markers over three months, and giving them oral calcium reversed the drop [1]. That is the clearest mechanism we have, and it is why the FDA label specifically notes that people with inadequate B12 or calcium intake seem more prone to the effect [2].

Two things follow from this that people get wrong:

• It is not a chemical clash. Metformin and B12 do not react with each other in your stomach. Taking them at different times of day does not fix the problem, because the problem is at the absorption step in your gut, not in the pill bottle.
• It is slow. Your liver stores enough B12 to last years. That is why the effect rarely shows up in month three and does show up in year four.

How likely is it to affect you

In people with diabetes (the strongest evidence):

• The FDA label reports that in 29-week trials, about 7% of patients dropped from normal to subnormal B12 [2].
• In the Diabetes Prevention Program Outcomes Study, one of the longest metformin trials ever run, low B12 (≤203 pg/mL) occurred in 4.3% of the metformin group vs 2.3% on placebo at 5 years. Counting borderline-low too, it was 19.1% vs 9.5%. Each additional year on metformin raised the odds of deficiency by about 13% [3].
• In a Dutch trial, metformin lowered B12 about 19% over 4.3 years, and roughly one in 14 people treated developed deficiency who otherwise would not have [4].
• In 2022 the UK medicines regulator reclassified low B12 as a common side effect of long-term or high-dose metformin, affecting up to 1 in 10 [5].
• The American Diabetes Association's 2026 Standards of Care recommend considering periodic B12 checks for anyone on long-term metformin, and specifically flag higher doses (1,500 mg/day or more), longer duration (4 to 5 years), anemia, nerve symptoms, or kidney disease as reasons to look harder [6].

In women with PCOS (the evidence is thinner, and we will not pretend otherwise):

• A 2022 study compared 52 women with PCOS who had been on metformin for at least six months with 50 newly diagnosed women who had never taken it. The metformin group had lower B12 (median 272 vs 385 pg/mL), and 57.7% had deficient or borderline levels compared with 24% of the untreated group [7]. Note the untreated number: about one in four newly diagnosed women already had low-ish B12 before any metformin, which matches an older finding that B12 runs lower in women with PCOS and insulin resistance to begin with [8].
• Short trials of 3 to 6 months in PCOS show mixed results, and a 2021 meta-analysis pooling them found no significant change in B12 over that timeframe [9]. One careful study found that metformin lowered total B12 in the blood but did not change the markers that reflect what your cells can actually use [10]. That is a real caveat, and it is part of why PCOS guidelines say "consider monitoring" rather than "everyone must supplement."
• The 2023 International PCOS Guideline, the reference document for how PCOS is treated, recommends metformin for adults with PCOS and BMI ≥25, and states in its practice notes that metformin use may be associated with low B12, especially where other risk factors exist, and that monitoring should be considered [11].

Bottom line on likelihood: if you are in your first year on a moderate dose with a normal diet, your risk is low. If you have been on 1,500 mg or more for several years, the diabetes data are the best guide we have, and they say roughly one in five people will be low or borderline. PCOS does not make you immune to that; if anything, the baseline is worse.

Why this matters more if you have PCOS

Three PCOS-specific reasons this is not just a diabetes footnote:

1. You may be starting lower. As above, low B12 is more common in women with PCOS before metformin enters the picture [7, 8].

2. Homocysteine. B12 (with folate and B6) is needed to clear homocysteine, an amino acid linked to cardiovascular risk. In a randomized trial of 60 women with PCOS, three months of metformin at 850 mg twice daily raised homocysteine by about 26%, and adding B-group vitamins or folic acid brought it back down [12]. Women with PCOS already carry higher long-term cardiometabolic risk, so this is worth knowing.

3. Fertility and pregnancy. Many women with PCOS are on metformin specifically while trying to conceive. The RDA for B12 rises from 2.4 mcg to 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding [13]. B12 and folate work together in early fetal development. If you are taking a prenatal, you are almost certainly covered on both. If you are taking folic acid alone (not a full prenatal) while on metformin, that is the one combination worth raising with your clinician, because high folate intake can correct the blood-count sign of B12 deficiency while the deficiency itself continues [14].

Symptoms that should prompt a test now

B12 deficiency is easy to miss because its early signs overlap with PCOS itself and with plain exhaustion:

• Fatigue that does not track with sleep
• Tingling, numbness, or "pins and needles" in hands or feet
• Low mood, brain fog, or memory slips
• A sore or unusually smooth tongue
• Pale skin or shortness of breath on exertion (anemia)

Any of these on long-term metformin is a reason to ask for a B12 level this month, not at your next annual.

What to actually do

If you are starting metformin: ask for a baseline B12. It costs almost nothing and gives you a number to compare against later.

If you have been on it a while and never been tested: ask for a B12 level. The FDA label says every 2 to 3 years as a floor [2]. Move to annual if any of these apply: dose ≥1,500 mg/day, 4+ years of use, symptoms above, a proton-pump inhibitor or H2 blocker (omeprazole, pantoprazole, famotidine and similar, which independently lower B12 [13]), vegan or vegetarian diet, prior bariatric or gut surgery, or you are trying to conceive.

If your result comes back borderline (roughly 200 to 300 pg/mL, or 148 to 221 pmol/L): ask whether a methylmalonic acid (MMA) or holotranscobalamin test is available. These tell you whether the low number actually reflects what your cells are getting [10]. Lab reference ranges vary; use your lab's.

If it comes back low:

• Do not stop metformin over this. The deficiency is correctable; the reasons you are on metformin usually are not.
• Oral B12 works. In the trials that compared them, 1,000 mcg a day by mouth restored blood levels about as well as injections, and 2,000 mcg a day may do even better [15]. High oral doses work even when the intrinsic-factor route is impaired, because a small fraction is absorbed by simple diffusion. Injections are for people who cannot absorb orally at all, or who have neurological symptoms that need fast correction.
• Recheck in about 3 months to confirm it worked, then fall back to the schedule above.

On calcium: the mechanism study used about 1.2 g/day of oral calcium to reverse the absorption block [1]. That does not mean you should add a calcium supplement just for this. It does mean that if your calcium intake is low, fixing that is a reasonable first step, and if you are already taking calcium for another reason, it may be helping here too.

What we do not recommend

This is the part most supplement content skips, so we will be explicit.

• You do not need a "metformin-specific" B12 formula. There is no such thing clinically. A plain cyanocobalamin or methylcobalamin tablet at 1,000 mcg is the intervention studied. We are not aware of good evidence that methylcobalamin outperforms cyanocobalamin for correcting deficiency [16]; pick whichever you tolerate and can afford.
• You do not need injections as a first step unless you have neurological symptoms or a diagnosed absorption disorder like pernicious anemia [15].
• You do not need a "homocysteine-lowering stack." If your homocysteine is high and your B12 is low, correcting the B12 is the fix. A prenatal or a basic B-complex covers folate and B6.
• You do not need to take B12 at a different time from metformin. See mechanism above. Timing tricks are a solution to a problem this interaction does not have.
• There is no established upper limit for B12 because excess is excreted rather than stored [13], so high oral doses are not the safety concern here. The concern is the untreated deficiency.

How confident are we

When to talk to a clinician rather than read further

• You have tingling or numbness that is new
• You are pregnant or trying to conceive and have not had B12 checked on metformin
• You have been on metformin 5+ years with no B12 test on record
• You have a history of gut surgery, celiac disease, or pernicious anemia
• You take a PPI or H2 blocker daily alongside metformin

None of this replaces your doctor. It is meant to make the conversation with them faster and better.

Quick questions

Can I take B12 with metformin?

Yes. If you have been on metformin for a while, you may actually need to. Metformin lowers how much B12 your body absorbs. B12 does not change how metformin works.

Does metformin cause B12 deficiency?

Metformin lowers how much B12 your body absorbs. It is slow: your liver stores enough B12 to last years, so the effect rarely shows up in month three and does show up in year four. If you have been on 1,500 mg or more for several years, the diabetes data are the best guide we have, and they say roughly one in five people will be low or borderline.

Should I take B12 at a different time than metformin?

No. Metformin and B12 do not react with each other in your stomach. Taking them at different times of day does not fix the problem, because the problem is at the absorption step in your gut, not in the pill bottle.