Yes, methotrexate is classified as an immunosuppressant. It’s also categorized as an antimetabolite and a disease-modifying antirheumatic drug (DMARD), depending on the condition it’s being used to treat. This triple classification reflects the fact that methotrexate works differently at different doses, but its ability to dial down immune activity is central to most of its uses.
How Methotrexate Suppresses the Immune System
Methotrexate doesn’t shut down the immune system the way most people imagine. Rather than broadly disabling your defenses, it works through several targeted mechanisms that reduce inflammation and slow overactive immune responses.
The most widely accepted explanation involves adenosine, a naturally occurring molecule that acts as a chemical brake on inflammation. Methotrexate increases adenosine levels outside your cells. When adenosine binds to receptors on immune cells, it triggers a cascade of anti-inflammatory effects: neutrophils (your first-responder white blood cells) become less aggressive, macrophages shift from a pro-inflammatory state to a calmer one, and T cells become less likely to activate and multiply. This is why methotrexate can quiet the joint destruction in rheumatoid arthritis without leaving you as vulnerable to infection as stronger immunosuppressants might.
Methotrexate also blocks an enzyme called dihydrofolate reductase, which cells need to produce DNA building blocks. By starving rapidly dividing immune cells of these materials, it reduces both the number and activity of T cells driving autoimmune attacks. Studies in human T cells show that methotrexate lowers levels of ATP and GTP (two key energy molecules), which slows T cell proliferation and pushes some toward programmed cell death. On top of that, methotrexate reduces levels of adhesion molecules on blood vessel walls, making it harder for inflammatory cells to migrate into joints. Biopsies of joint tissue after 16 weeks of treatment show measurable drops in inflammatory cell counts and inflammatory signaling molecules like TNF and IL-1.
Why the Dose Changes Everything
Methotrexate behaves like two different drugs depending on the dose. For autoimmune conditions like rheumatoid arthritis, the typical starting dose is 7.5 milligrams once a week. Psoriasis patients may start at 10 to 25 milligrams per week, with a cap around 30 milligrams. At these low doses, methotrexate primarily works as an immunosuppressant and anti-inflammatory agent through the adenosine pathway described above.
In cancer treatment, the math changes dramatically. Doses for acute lymphoblastic leukemia, for example, are calculated by body surface area and given more frequently. At these higher doses, methotrexate acts mainly as an antimetabolite, starving cancer cells of the folate they need to copy their DNA and divide. The immunosuppressive effects still occur, but the primary goal shifts to killing rapidly growing cells.
What This Means for Infection Risk
Because methotrexate tamps down immune function, infection risk is a reasonable concern. The actual numbers, though, are more reassuring than many patients expect. A large systematic review and meta-analysis found that methotrexate increased infection risk by about 25% in people with rheumatoid arthritis. In practical terms, that translates to roughly 88 additional infections per 1,000 patients treated, compared to placebo. For people taking methotrexate for other inflammatory conditions, the excess risk was negligible: only about 7 additional infections per 1,000 patients.
Notably, there was no increased risk of serious infections across all inflammatory disease populations studied, and no increased risk of respiratory infections specifically. The infection risk also didn’t climb with higher doses. Patients on doses above 15 milligrams per week actually showed no statistically significant increase in infections, while those at 15 milligrams per week showed a borderline increase. This counterintuitive finding likely reflects the complex relationship between controlling inflammation (which itself raises infection risk) and the immunosuppressive effects of the drug.
What Methotrexate Treats
The American College of Rheumatology considers methotrexate the preferred first-line DMARD for rheumatoid arthritis, ranking it above alternatives like sulfasalazine or hydroxychloroquine. Beyond RA, it’s approved for psoriasis and polyarticular juvenile idiopathic arthritis. Its immunosuppressive properties also make it useful for organ transplant recipients, where preventing immune rejection of the donated organ is critical.
On the oncology side, methotrexate treats several cancers including acute lymphoblastic leukemia, non-Hodgkin lymphoma, and mycosis fungoides (a type of skin lymphoma). The same basic chemistry applies in every case: methotrexate interferes with folate metabolism. The difference is scale and intent.
Folic Acid and Side Effect Management
Because methotrexate works by blocking folate pathways, most prescribers recommend folic acid supplements to reduce side effects like nausea, mouth sores, and liver enzyme elevations. Guidelines generally suggest 5 to 10 milligrams of folic acid per week, taken on the days you’re not taking methotrexate. The practice of skipping folic acid on methotrexate day isn’t strongly evidence-based, but it’s a common recommendation based on the logic that you don’t want to counteract the drug on the day it’s most active.
Monitoring While on Methotrexate
Long-term methotrexate use requires regular blood work, primarily to watch for liver damage. The standard protocol calls for monthly liver enzyme checks for at least the first six months, then every three months after that. If liver enzymes rise above three times the normal upper limit and stay there, your prescriber will typically pause or stop the medication.
Blood counts also need monitoring since methotrexate can suppress bone marrow production of blood cells, another consequence of its antimetabolite activity. This is separate from its immunosuppressive effects but equally important to track.
Alcohol and Pregnancy Considerations
Alcohol and methotrexate both stress the liver, so combining them raises the risk of liver damage. The American College of Rheumatology recommends abstinence from alcohol with only occasional exceptions. British guidelines take a somewhat more relaxed approach, advising patients to stay well within the general population limit of 14 units per week. In practice, most rheumatologists land somewhere in between: occasional light drinking may be acceptable, but regular consumption is discouraged.
Methotrexate is firmly contraindicated during pregnancy because it disrupts DNA synthesis in rapidly dividing fetal cells. The drug label recommends waiting 3 to 6 months after stopping methotrexate before trying to conceive, though some providers consider 1 to 3 months sufficient to clear the medication from the body. This waiting period applies to both men and women.