what are all the side effects of methotrexate

Methotrexate is a chemotherapy drug repurposed at lower doses for autoimmune disease, and its side effects are serious enough that the evidence here shows about 1 in 9 people cannot tolerate it at all.

The evidence comes from small, mostly open-label studies, not from large manufacturer-funded trials with hard endpoints. The 1986 long-term study of 29 rheumatoid arthritis patients found that 90% had some adverse event, with gastrointestinal problems the most common (18 of 29 patients) and oral ulcers in 11 of 29 [4]. A 2013 cross-sectional study of 291 patients found 11% met criteria for methotrexate intolerance, meaning the side effects were bad enough to score high on a validated scale [1]. That rate was much higher with injections (20.6%) than with pills (6.2%) [1].

The most dangerous side effect is lung damage. A 2014 systematic review found 32 reported cases of interstitial lung disease linked to methotrexate in rheumatoid arthritis patients, with 26 of those being acute [5]. This is not a rare curiosity. It is a known, documented toxicity that can be fatal. The 1986 study also reported one case of hypersensitivity pneumonitis, a severe lung inflammation [4].

Liver toxicity is real but the evidence is mixed on how often it causes permanent damage. In the 1986 study, 20 of 29 patients had elevated liver enzymes at some point, but serial liver biopsies over two years did not show clear progression of scarring [4]. That is a small study with short follow-up. The 2020 trial of 80 patients found liver dysfunction in 4 of 40 patients on methotrexate, described as "slightly elevated liver enzymes" [3].

Gastrointestinal side effects dominate the patient experience. Nausea, vomiting, abdominal discomfort, and diarrhea are common. A 2014 study of 37 patients who switched from oral to subcutaneous methotrexate found that nausea frequency dropped from a mean VAS of 36.5 to 25.0, and nausea intensity from 34.9 to 22.6, both statistically significant [2]. That means the oral form is harder on the gut.

Bone marrow suppression is another documented risk. The 1986 study reported leukopenia (low white blood cells) in 6 of 29 patients [4]. The 2020 trial found leukocyte reduction in 3 of 40 control patients on methotrexate [3]. This matters because it leaves you vulnerable to infection.

The evidence here does not cover long-term cancer risk, fertility effects, or neurological side effects. Those are real concerns with any drug that interferes with cell division, but the retrieved studies do not address them. The absence is not safety.

My call: methotrexate has a high rate of side effects that are real, documented, and sometimes serious. The evidence is moderate quality from small studies, not from large rigorous trials. The burden of proof is on the prescriber to justify why this drug is worth the risk for you, not on you to accept it. Confidence: moderate.