Methotrexate Side Effects: What to Expect and When to Call Your Doctor

Methotrexate is a powerful medication with a well-known side effect profile. Most side effects are manageable, especially at the low weekly doses used for arthritis and psoriasis. But some side effects are serious — and knowing which symptoms require immediate attention could be life-saving. Here's what every patient taking methotrexate should know.

How Common Are Side Effects With Methotrexate?

Between 7% and 30% of patients stop methotrexate within the first year due to side effects — and that number is higher at higher doses. However, many side effects can be prevented or reduced with folic acid supplementation, and most are reversible if caught early. Regular monitoring is essential throughout therapy.

Common Side Effects of Methotrexate

These side effects are frequently reported, especially in the first weeks of treatment. Most improve over time or with dose adjustments:

Nausea and upset stomach: Most common side effect; often occurs the day after taking the dose (known as "methotrexate hangover"). Taking it at night, with food, or switching to injection can help.

Fatigue: Tiredness on the day of and day after taking methotrexate is very common

Mouth sores (mucositis): Painful ulcers inside the mouth; often improved by taking folic acid daily

Loss of appetite: Particularly common in the days following a dose

Headache and dizziness: Usually mild; notify your doctor if severe

Hair thinning: Hair loss is possible but generally less severe than with chemotherapy doses; often resolves after stopping

Sun sensitivity: Use broad-spectrum sunscreen (SPF 30+) and avoid prolonged sun exposure while on methotrexate

How Folic Acid Reduces Methotrexate Side Effects

Folic acid (vitamin B9) is almost universally prescribed alongside low-dose methotrexate for autoimmune conditions. Taking folic acid 1 mg daily (on the days you don't take methotrexate, though daily is also common) significantly reduces the risk of mouth sores, GI symptoms, and hair loss without reducing methotrexate's therapeutic effect. Folinic acid (leucovorin) is used for rescue in high-dose chemotherapy settings.

Serious Side Effects — Call Your Doctor Immediately

These side effects are less common but potentially life-threatening. Do not wait for your next appointment if you experience any of the following:

Liver toxicity signs: Yellowing of skin or eyes (jaundice), dark urine, severe abdominal pain, unusual fatigue, swelling in the legs — seek evaluation immediately. Long-term use can cause hepatic fibrosis or cirrhosis.

Lung toxicity (methotrexate pneumonitis): New dry cough, shortness of breath, or fever — can occur at any dose and is potentially fatal if not treated promptly

Bone marrow suppression: Unusual bruising or bleeding, frequent infections, extreme fatigue, fever — low white blood cell count leaves you vulnerable to serious infection

Kidney problems: Significant changes in urinary output, swelling, or lab values — methotrexate is excreted by kidneys and builds up if kidneys are impaired

Serious skin reactions: Severe rash, blistering, or peeling skin — can indicate Stevens-Johnson syndrome, a medical emergency

Signs of serious infection: High fever, chills, sweats — methotrexate suppresses the immune system and can mask infection symptoms

Why Regular Blood Tests Are Essential

The most serious side effects — liver toxicity and bone marrow suppression — often cause no symptoms until they are severe. That's why monitoring labs are non-negotiable:

CBC (complete blood count) and liver function tests every 1–2 weeks when starting or changing dose

Once stable: CBC and LFTs every 2–3 months

Renal function (creatinine) monitored regularly — especially important in older patients or anyone with kidney disease

The Critical Dosing Warning: Weekly, Not Daily

Methotrexate for RA, psoriasis, and other non-cancer uses is taken ONCE WEEKLY — not every day. Fatal overdoses have occurred when patients accidentally took their weekly dose daily. This is a boxed warning on the label. Always confirm your dosing schedule with your provider, and write it clearly on your medication bottle.

For more important safety information, read about methotrexate drug interactions and our overview of what methotrexate is used for.