If you have rheumatoid arthritis (RA), your doctor may have prescribed methotrexate for treatment.

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 manufactured 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 (RBCs) and other healthy cells. It’s also necessary for DNA growth and repair.

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

Although it’s good for you to get folate by eating a variety of these foods, simply eating more of these foods won’t 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. Some symptoms caused by folate deficiency include:

Folic acid is the manufactured 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, which are taken orally, can help decrease the side effects from folate deficiency. They’re available for purchase over the counter, either online or at your local drugstore.

The usual starting dosage is 1 milligram (mg) per day. Your doctor will determine a dosage of folic acid that’s right for you.

Taking folic acid with methotrexate doesn’t decrease methotrexate's effectiveness in treating your RA.

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.

RA is an autoimmune disorder. Autoimmune disorders occur when your immune system mistakes your body’s tissues for invaders and attacks them.

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. Learn more about treatments for RA.

Sometimes methotrexate 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.

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