Quick answer

Ask your pharmacist or doctor before combining. The methotrexate label names folic acid and folinic acid, not the other B vitamins: for cancer, take them only if your provider directs; for arthritis and psoriasis, prescribers are told to give them. A B complex may or may not contain folic acid, so the Supplement Facts panel decides.^1,2

Key facts

The methotrexate label names folic acid and folinic acid, and products with folic acid or its derivatives. It does not name vitamin B12 or the other B vitamins.¹
NIH's fact sheets for vitamin B12, B6, niacin, thiamin, riboflavin and biotin do not mention methotrexate. Its folate sheets do.^{2,3,4,5,6,7,8}
MedlinePlus says that when methotrexate is taken for psoriasis or arthritis, your doctor will probably tell you to take folic acid to decrease some of its side effects.⁹
A B complex combines several of the eight B vitamins, and products differ in which ones they include.^3,10

Ask your pharmacist or doctor first

The methotrexate label says products containing folic acid may make methotrexate less effective when it is used for cancer, and that patients should take them only when their healthcare provider directs. For other uses, the label tells prescribers to give folic acid or folinic acid to lower the chance of side effects.^1,2

On the medicine's label

Which B vitamin in a B complex matters with methotrexate?

Folate, the vitamin known as B9, is the one the sources name. The methotrexate label describes the medicine as an inhibitor of dihydrofolate reductase, the enzyme cells use to put folate to work, and says it competes with reduced folates to enter cells. That mechanism fits the label's focus on folic acid and its derivatives. The label says nothing similar about thiamin, riboflavin, niacin, B6, biotin or B12.¹

Products differ in which B vitamins they include and how much of each they contain, so the Supplement Facts panel is the only way to know what a given B complex supplies.^3,10

So the question is less whether a product is called a B complex and more whether yours lists folic acid or another form of folate.

What does the methotrexate label say about folic acid?

It treats two groups differently. For people taking methotrexate for cancer, it says products containing folic acid or its derivatives may decrease methotrexate's clinical effectiveness, and tells patients not to take them unless their healthcare provider directs it. For rheumatoid arthritis, psoriasis and polyarticular juvenile idiopathic arthritis, it says folate deficiency may increase side effects and tells prescribers to give folic acid or folinic acid.¹

A B complex that carries folic acid can therefore be the thing to avoid, or a duplicate of what your prescriber already gave you. Which one it is depends on why you take methotrexate, and no page can decide that for you. Your prescriber decides whether you need folic acid or folinic acid, how much and on which days.^1,9

Can you take B12 with methotrexate?

The label and the MedlinePlus page name folic acid, and NIH's vitamin B12 sheet does not mention methotrexate. The sources checked do not address B12 on its own. That is not a finding either way, so ask your pharmacist. A B complex that includes both B12 and folic acid is a different question from a B12 product alone.^1,3,9

What to bring when you ask

Do I take methotrexate for cancer or for another condition, and does that change the answer on folic acid?
Does my B complex list folic acid, folate or another form, and does the label's wording about derivatives cover it?
Should I keep taking, change or pause the B complex while we decide?
Does my prescriber already give me folic acid or folinic acid?

MedlinePlus advises keeping a written list of all prescription and nonprescription medicines, vitamins, minerals and dietary supplements, and bringing it to every appointment. The methotrexate label's patient information asks people to tell their provider about vitamins and herbal supplements too.^1,9

Questions people ask

Can I take vitamin B complex with methotrexate?

Ask first. For cancer, the label says to take folic acid products only as your provider directs; for arthritis and psoriasis it tells prescribers to give folic acid. Which applies depends on why you take methotrexate.¹

Can I take B12 with methotrexate?

The label and MedlinePlus name folic acid, not B12, and NIH's B12 sheet does not mention methotrexate. The sources checked do not address B12 alone, so ask your pharmacist.^1,3,9

Can you take B vitamins with methotrexate?

Only folic acid and folinic acid are named by the label. Because a B complex mixes several B vitamins, check the panel for folic acid and ask your pharmacist about the rest.¹

Does a B complex contain folic acid?

Some do and some do not. Products differ in which B vitamins they include and how much of each, so the Supplement Facts panel is the only way to tell.^3,10

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Sources

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