Can I Take Methotrexate With an MTHFR Gene Variant?

No, having a common MTHFR gene variant (often referred to as a mutation) does not mean you must avoid taking methotrexate. While some genetic variants may be associated with a slightly higher chance of side effects in studies, they do not reliably predict how your individual body will react. Major pharmacogenomic guidelines—including those from the Clinical Pharmacogenetics Implementation Consortium (CPIC) and the Dutch Pharmacogenetics Working Group (DPWG) (organizations that guide how genetics affect drug responses)—do not consider common MTHFR variants to be a reason to avoid the drug or to automatically change your dose ^[1][2].

If a genetic test (such as 23andMe or a report from a healthcare provider) shows you have the C677T or A1298C variants, it is important to understand that these are very common in the general population. They are not the same as a rare, severe MTHFR deficiency disease, and they do not mean your body is unable to process ordinary folic acid ^[3]. Your doctor will base your treatment on your kidney function, liver function, other medications, and overall health—clinical factors that matter far more for your safety than your MTHFR genetic status.

Safe Dosing and Crucial Warning Signs

When prescribed for inflammatory conditions like rheumatoid arthritis, methotrexate is taken exactly once weekly—not daily. Taking it daily by mistake can cause life-threatening toxicity. Never double your dose if you miss one, and contact your doctor immediately if you accidentally take an extra dose.

When to Seek Medical Help:
While taking methotrexate, you must actively watch for signs of toxicity. Do not wait for your next appointment. Contact your prescriber urgently or seek emergency care if you develop:

• Severe or persistent mouth sores (ulcers)
• Fever, chills, or sore throat (signs of an infection)
• Unusual bruising or bleeding
• Severe or persistent vomiting or diarrhea
• Yellowing of your skin or eyes (jaundice) or dark urine (signs of liver damage)
• A severe skin rash or blistering
• A new, dry cough or unusual shortness of breath

What is the MTHFR Gene?

The MTHFR gene provides instructions for making an enzyme that helps your body process folate (vitamin B9) into an active form that your cells can use ^[3]. Methotrexate works by interfering with how your body uses folate, which helps suppress an overactive immune system.

The two most common variants are C677T and A1298C. Inheriting one or two copies of these variants can slightly reduce the activity of the MTHFR enzyme ^[3][4].

Does the Variant Increase Toxicity Risk?

The relationship between MTHFR variants and methotrexate side effects is mixed and depends on the specific variant:

It is crucial to note that many studies reporting high toxicity risks evaluated the massive doses of methotrexate used in cancer treatments. Those risks cannot be directly applied to the much lower weekly doses used in rheumatology ^[10][11].

Folic Acid and Routine Monitoring

Because methotrexate depletes folate, your doctor will routinely prescribe a folic acid supplement to help prevent side effects like mouth sores and nausea ^[12]. While folic acid helps reduce these risks, it does not make methotrexate entirely risk-free.

• Follow your exact schedule: Folic acid is usually taken daily or on specific days of the week, depending on your doctor’s protocol. Follow your exact prescription. Do not increase your dose, switch to a different supplement like methylfolate, or substitute it with folinic acid (a stronger prescription rescue medicine) just because of a genetic report ^[1][13].
• Routine blood tests: Regular laboratory testing is the safest and most reliable way to monitor for unseen toxicity. Before and during treatment, your doctor will check a complete blood count (CBC), liver tests, and kidney function ^[14].
• Baseline folate levels: Your doctor may test your actual folate and vitamin B12 levels in your blood if they suspect a deficiency based on your medical history or diet. Testing is not automatically required for everyone with an MTHFR variant.