If you have rheumatoid arthritis (RA), your doctor may have prescribed methotrexate for treatment. According to the American College of Rheumatology, methotrexate is one of the most commonly used drugs to treat RA. However, it can decrease the levels of an important vitamin in your body called folate. This leads to a side effect of methotrexate called folate deficiency. Your doctor may suggest you take a folic acid supplement, which is a man-made form of folate.

Folate is a B vitamin that has a role in many important functions in your body. It helps your body make new red blood cells and other healthy cells. It’s also necessary for DNA repair.

Folate can be found in many different foods. These foods include:

  • leafy vegetables such as spinach, broccoli, and lettuce
  • okra
  • asparagus
  • certain fruits such as bananas, melons, and lemons
  • legumes such as peas, beans, lentils, soybeans, and peanuts
  • mushrooms
  • organ meat such as beef liver and kidney
  • orange juice and tomato juice

Although it’s good for you to get folate by eating a variety of these foods, simply eating more of these foods will not be enough to make up for the folate that you lose from methotrexate.

Methotrexate interferes with the way your body breaks down folate. When you take methotrexate, you can develop levels of folate that are lower than normal. This is because methotrexate causes your body to get rid of more folate as waste than usual. This effect causes folate deficiency. Your doctor can prescribe the supplement folic acid to help prevent a folate deficiency.

Folic acid is the man-made form of folate. Taking folic acid can help to make up for, or supplement, the folate that your body loses when you take methotrexate. Folic acid supplements can help decrease the side effects from folate deficiency. They’re available over the counter and cost about as much as other vitamins and supplements.

These supplements come in a form that you take by mouth. The usual starting dosage is 1 mg per day. Your doctor will determine a dosage of folic acid that’s right for you.

Learn more: Folate vs. folic acid »

Taking folic acid with methotrexate does not decrease methotrexate's effectiveness in treating your rheumatoid arthritis. When you use methotrexate to treat RA, it helps decrease pain and swelling by blocking certain chemicals in your body that lead to inflammation. Methotrexate does block folate, but the way it treats RA seems to be mostly unrelated to blocking folate. Therefore, taking folic acid to make up for the folate you lose from taking methotrexate helps reduce the side effects of folate deficiency without affecting your treatment of RA.

Read more: Complete drug information for methotrexate »

RA is an autoimmune disorder. In RA, your immune system specifically attacks the synovium, which is the lining of the membranes that surround your joints. The inflammation from this attack causes the synovium to thicken. If you don’t treat your RA, this thickened synovium can lead to cartilage and bone destruction. The tissues that hold your joints together, called tendons and ligaments, can weaken and stretch. This can cause your joints to lose their shape over time, which can affect how well you can move around. The inflammation associated with RA can damage other parts of the body as well. These include your skin, eyes, lungs, heart and blood vessels. Treating your RA can decrease these effects and improve your quality of life.

Many doctors prescribe methotrexate to treat RA. Sometimes this drug leads to folate deficiency, which can cause some bothersome side effects. However, these side effects can often be avoided by taking folic acid. Treating your RA is very important, so making your treatment as easy as possible is important. If your doctor prescribes methotrexate for your RA, talk to them about your risk of folate deficiency and the possibility of using folic acid to prevent side effects.