Quick answer

Ask your pharmacist or doctor before combining. Official sources document 4 supplements for metformin: vitamin B12, B-complex, niacin and chromium. The metformin label says the medicine may lower vitamin B12 and lists nicotinic acid among drugs that can weaken blood sugar control, and NIH says chromium might add to its effect.^1,2,3

Key facts

In 29-week trials described on the metformin label, about 7% of people with normal vitamin B12 dropped below normal.¹
The B12 and B-complex entries are about monitoring, not avoidance: the label tells prescribers to measure B12 every 2 to 3 years.^1,4
The metformin label lists nicotinic acid, a form of niacin, among drugs that tend to raise blood sugar, and asks for close watching when one is started or stopped.²
NIH says chromium might add to the effect of metformin and raise the risk of low blood sugar, on the reasoning that some studies suggest it might lower blood glucose.³
MedlinePlus asks people on metformin to list every vitamin, supplement and herbal product they take, and to mention any past low vitamin B12 level.^5,6

What is metformin for, and why do vitamins come up?

MedlinePlus says metformin is used alone or with other medicines, including insulin, for type 2 diabetes. It lowers the amount of glucose absorbed from food and the amount made by the liver, and it increases the body's response to insulin.⁵

Two different supplement questions sit on this one medicine. One is a nutrient the medicine may deplete, vitamin B12. The other is a group of supplements that may push blood sugar up or down, which matters because controlling blood sugar is the whole point of taking metformin.^1,2,3

Does metformin lower vitamin B12, and what about a B-complex?

Your doctor may want to check on you

Metformin may lower vitamin B12 levels, possibly by interfering with its absorption. In 29-week trials described on the metformin label, about 7% of people with normal B12 dropped below normal, and the label tells prescribers to measure B12 every 2 to 3 years.^1,6,7,8

On the medicine's label

Your doctor may want to check on you

Metformin may lower vitamin B12 levels, possibly by reducing its absorption. The metformin label recommends measuring vitamin B12 every 2 to 3 years in people who take it.^4,9

On the medicine's label

These are the only two cards where the supplement is the thing at stake rather than the medicine. The sources checked tell prescribers to measure B12 and ask patients to mention past low levels. They do not tell patients to start, stop or change a supplement, and this page does not either. Whether a B12 supplement or a B-complex fits your situation is a question for the person who can order the test.^4,6,8

Which supplements can move blood sugar while you take metformin?

Ask your pharmacist or doctor first

Some studies suggest chromium might lower blood glucose. NIH says it might therefore add to the effect of metformin and other diabetes medicines and raise the risk of low blood sugar (hypoglycemia).^3,10

Expected from how the two work, not shown in studies

The chromium card is tagged expected, not on the label. NIH's wording is conditional, so the entry rests on its reading of studies and not on a line in the metformin label.^3,10

Your doctor may want to check on you

The metformin label lists nicotinic acid among drugs that tend to raise blood sugar and may lead to loss of blood sugar control, and advises close watching when such a drug is started or stopped. NIH says nicotinic acid at 1.5 g a day or more is most likely to raise blood sugar, and that people taking diabetes medicines may need their doses adjusted.^2,11,12

On the medicine's label

The niacin card points the other way, and it does rest on the metformin label. Nicotinic acid is the form of niacin that causes flushing, and NIH names the highest amounts as the ones most likely to raise blood sugar.^2,11

One entry says a supplement might lower glucose and the other says a supplement might raise it. Having them side by side shows why a pharmacist needs the whole list instead of one supplement at a time.

What do the monitor and on the label tags mean for metformin?

Three of the four cards are tagged monitor, and all three are on the label: the two B12 entries and the niacin entry. The chromium card alone is tagged ask first and expected. A monitor tag means a source asks for something to be watched, such as a B12 level or blood sugar after a change, instead of describing a combination to stay away from.

The four documented supplements for metformin
Supplement	What the sources describe	How it is known
Vitamin B12	Metformin may lower B12 levels	On the label
B-complex	Same concern, because it contains B12	On the label
Niacin	Nicotinic acid may raise blood sugar	On the label
Chromium	Might add to metformin's effect on blood sugar	Expected from how the two work

Questions to ask your pharmacist about metformin and supplements

Should my vitamin B12 be checked, and when was it last measured?¹
Does my multivitamin, B-complex or protein powder contain niacin or nicotinic acid?
If I take chromium, should I check my blood sugar more often?³
Which signs of low and high blood sugar should I watch for?⁵

For a supplement that is not on this page, the sources checked do not address it. That is not a finding either way, so ask your pharmacist.

Questions people ask

What supplements should not be taken with metformin?

Official sources document 4: vitamin B12, B-complex, niacin and chromium. None is tagged avoid. The label asks for B12 monitoring and for close watching of blood sugar with nicotinic acid, and NIH says chromium might add to metformin's effect.^1,2,3

What vitamins not to take with metformin?

The documented vitamins are B12, the B-complex that contains it, and niacin. The label concern for B12 is that metformin may lower it, which is a reason to measure it. Ask your pharmacist before you start or stop any of them.^1,2

What supplements interfere with metformin?

The metformin label names nicotinic acid among drugs that can weaken blood sugar control, and NIH says chromium might add to the medicine's effect. Vitamin B12 is the reverse case, because the medicine may lower the vitamin.^1,2,3

Which vitamins to avoid with metformin?

No entry on this page tells you to avoid a vitamin outright. The label ties the medicine to lower B12 and asks for monitoring, and it lists nicotinic acid among drugs that can raise blood sugar. Ask your pharmacist about both.^1,2

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.