Methotrexate is officially classified as an immunosuppressant, but that label tells only part of the story. At the low doses used for autoimmune conditions like rheumatoid arthritis, methotrexate acts more as an anti-inflammatory and immunomodulator than a drug that broadly shuts down the immune system. The distinction matters because it shapes how much infection risk you actually face, whether vaccines will work for you, and why the drug remains the first-line treatment for rheumatoid arthritis decades after it was repurposed from cancer therapy.
How Methotrexate Works at Low Doses
Methotrexate was originally designed to block an enzyme called dihydrofolate reductase, which cells need to make DNA building blocks. At the high doses used in cancer treatment, this action kills rapidly dividing cells, including immune cells. That is straightforward immunosuppression. But at the much lower weekly doses prescribed for rheumatoid arthritis and other autoimmune diseases, a different mechanism appears to dominate.
At low doses, methotrexate triggers a buildup of a molecule called AICAR inside cells, which in turn raises levels of adenosine both inside and outside the cell.1Joint Bone Spine. Review Methotrexate mechanism in treatment of rheumatoid arthritis Adenosine is an anti-inflammatory signaling molecule. When methotrexate boosts adenosine release from connective tissue cells like fibroblasts and endothelial cells, the result is a dampening of inflammation rather than a wholesale destruction of immune cells.2PubMed Central. Methotrexate inhibits neutrophil function by stimulating adenosine release from connective tissue cells In lab experiments, methotrexate increased adenosine release from fibroblasts roughly eightfold and dramatically reduced the ability of neutrophils to stick to those cells. Neutrophils are among the first immune cells to swarm an inflamed joint, so blocking their adhesion calms the damage without wiping out the cells entirely.
This is why researchers have long debated whether the label “immunosuppressant” even fits low-dose methotrexate. A review published in the early 1990s concluded that most available data pointed to methotrexate acting on the nonspecific inflammatory response rather than directly targeting T cells, the usual hallmark of immunosuppression.3PubMed Central. Methotrexate: anti-inflammatory or immunosuppressive? The drug does affect the immune system, but its primary therapeutic effect at weekly doses of 7.5 to 25 mg comes from quieting inflammation, not from suppressing immune surveillance the way drugs like cyclosporine or tacrolimus do.
What It Does to Inflammatory Signaling
One of the clearest ways methotrexate earns its reputation as a disease-modifying drug is by shifting the balance of inflammatory messenger molecules called cytokines. Studies in people with rheumatoid arthritis show that methotrexate reduces levels of several pro-inflammatory cytokines, including IL-1, IL-2, IL-6, and interferon-gamma, while nudging up levels of anti-inflammatory cytokines like IL-4 and IL-10.4PubMed. The influence of methotrexate on the gene expression of the pro-inflammatory cytokine IL-12A in the therapy of rheumatoid arthritis In a long-term study tracking patients over two and a half years, blood levels of IL-6, IL-8, and IL-1 beta all dropped consistently after starting methotrexate and stayed down.5BMJ. Long-term study of the impact of methotrexate on serum cytokines and lymphocyte subsets in patients with active rheumatoid arthritis
This cytokine rebalancing is different from what happens with classic immunosuppressants, which tend to broadly suppress T-cell activation. Methotrexate leaves large parts of the adaptive immune system relatively intact while dialing down the inflammatory cascade that drives joint destruction. That is good news for patients who worry about being “immunocompromised,” though it does not mean the drug is risk-free.
Does Methotrexate Raise Your Infection Risk?
The most practical question people have when they hear “immunosuppressant” is whether they will catch every cold going around or face dangerous infections. The evidence is more reassuring than many expect, though not entirely clean. A systematic review and meta-analysis found that methotrexate was associated with a modestly increased risk of infection in people with rheumatoid arthritis, roughly 25% higher than in controls. But that elevated risk did not hold up in people taking methotrexate for other inflammatory diseases, and there was no statistically significant increase in the overall rate of infections or serious infections across all conditions studied.6PubMed Central. Risk of Infection with Methotrexate Therapy in Inflammatory Diseases: A Systematic Review and Meta-Analysis
A large randomized trial looking at low-dose methotrexate in a different population (people with heart disease) found a small but real uptick in infectious side effects compared to placebo.7PubMed Central. Adverse Effects of Low-Dose Methotrexate: A Randomized Trial So the risk exists, but the size of it is modest for most people on standard low doses.
The scarier scenario is opportunistic infections, the kind caused by organisms a healthy immune system would normally keep in check. Case reports document opportunistic infections in people on low-dose methotrexate even when their white blood cell counts looked normal. Among documented cases, Pneumocystis pneumonia was the most common, accounting for nearly half of reported opportunistic infections.8PubMed. Opportunistic infection during treatment with low dose methotrexate These events are uncommon, but they underscore an important point: methotrexate does exert real effects on immune function that standard blood tests do not always flag.
Bone Marrow Suppression
Because methotrexate interferes with folate metabolism, it can suppress bone marrow function, reducing the production of red blood cells, white blood cells, and platelets. At low doses this is usually mild, but occasionally it hits hard and fast. One case report described a 76-year-old man who developed severe pancytopenia, with dangerously low neutrophil and platelet counts, within just three weeks of starting 15 mg per week of methotrexate. His blood smear showed clear signs of bone marrow suppression.9PubMed Central. A Rare Case of Acute Methotrexate Toxicity Leading to Bone Marrow Suppression Kidney problems contributed in that case, as impaired kidneys cannot clear methotrexate normally, allowing levels to build up.
Low-dose methotrexate carries a moderately increased risk of anemia and leukopenia more broadly, though severe myelosuppression remains uncommon.10PubMed Central. Adverse Effects of Low-Dose Methotrexate in a Randomized Double-Blind Placebo-Controlled Trial: Adjudicated Hematologic and Skin Cancer Outcomes in the Cardiovascular Inflammation Reduction Trial This is why your doctor orders regular blood work when you are on methotrexate. Catching a downward trend in blood counts early allows for dose adjustment before the situation becomes dangerous. People with kidney impairment, older adults, and those who are folate-depleted at baseline face higher risk.
Vaccines and Methotrexate
Even if methotrexate does not dramatically suppress infection-fighting immunity day to day, it clearly blunts the immune system’s ability to respond to vaccines. This became a pressing concern during the COVID-19 pandemic. A randomized controlled trial found that people who paused methotrexate for two weeks after receiving a COVID-19 booster produced roughly twice the antibody levels compared to those who kept taking it.11PubMed. Temporary 2-week suspension of methotrexate treatment to enhance COVID-19 vaccine response in people with immune-mediated inflammatory diseases That advantage held up at 12 weeks and was still detectable at 26 weeks. The trial also showed improved virus-neutralizing capacity, meaning the antibodies produced were functionally better, not just more abundant.
This finding has changed clinical practice. Many rheumatologists now recommend a brief methotrexate pause around vaccination, especially for flu shots and COVID-19 boosters. The two-week window is short enough that most people’s disease stays controlled, while the immune benefit is substantial. Live vaccines, on the other hand, remain generally contraindicated while on methotrexate because of the theoretical risk that a weakened live virus could cause actual infection in someone whose immune system is partly suppressed.
Lymphoproliferative Disorders and Immune Surveillance
One of the more unsettling risks of long-term methotrexate use is its association with lymphoproliferative disorders, a group of conditions where lymphocytes grow uncontrollably, sometimes resembling lymphoma. These are classified among “iatrogenic immunodeficiency-associated lymphoproliferative disorders,” a name that acknowledges the role of drug-induced immune suppression.12PubMed Central. Methotrexate-associated lymphoproliferative disorder demonstrating composite lymphoma of EBV-negative diffuse large B-cell lymphoma and EBV-positive mucocutaneous ulcer
Many of these cases are linked to Epstein-Barr virus. Roughly 30 to 50 percent of methotrexate-associated lymphoproliferative disorders test positive for EBV, and about half arise in locations outside the lymph nodes, such as the mouth, skin, or gut.13Human Pathology: Case Reports. Methotrexate-induced EBV-associated lymphoproliferative disorder presenting with an oropharyngeal mass The good news is that in many patients, simply stopping methotrexate leads to regression of the abnormal growth without chemotherapy. This suggests that methotrexate weakens immune surveillance of EBV just enough to let virus-driven cell proliferation get a foothold, and restoring normal immune function can reverse it. It also provides some of the strongest evidence that methotrexate does, in fact, suppress at least one arm of immune surveillance, even at low doses and even when the person seems otherwise immunologically healthy.
Why Folic Acid Matters
Methotrexate works in part by interfering with folate metabolism, and many of its side effects stem from that same interference. Folic acid supplementation is now standard practice alongside methotrexate, and the evidence behind it is solid. A Cochrane systematic review found that taking folic acid or folinic acid alongside methotrexate reduced gastrointestinal side effects like nausea and vomiting by about a quarter and slashed the risk of liver enzyme elevations by roughly 77%.14PubMed Central. Folic acid and folinic acid for reducing side effects in patients receiving methotrexate for rheumatoid arthritis Patients on folic acid were also far less likely to quit methotrexate altogether due to side effects.
The critical question is whether supplementing folate undermines methotrexate’s therapeutic benefit. The evidence says no. Multiple studies, including the Cochrane review, found no statistically significant reduction in methotrexate’s ability to control rheumatoid arthritis when patients took folic acid.15PubMed. The effect of folic acid supplementation on the toxicity of low-dose methotrexate in patients with rheumatoid arthritis This aligns with the understanding that methotrexate’s main anti-inflammatory action at low doses comes through the adenosine pathway rather than through folate depletion. You are essentially replenishing the folate that healthy cells need without interfering with the drug’s disease-modifying mechanism.
High-Dose Versus Low-Dose: Two Different Drugs in Practice
Much of the confusion about whether methotrexate is an immunosuppressant comes from the enormous range of doses used. For cancer treatment, methotrexate is given at doses hundreds or even thousands of times higher than what rheumatology patients take. At those doses, it is unambiguously cytotoxic and profoundly immunosuppressive, killing rapidly dividing cells including immune cells and requiring rescue with leucovorin to prevent life-threatening toxicity.
At the low doses used for rheumatoid arthritis, psoriasis, and other autoimmune conditions, typically 7.5 to 25 mg once per week, the drug behaves very differently. The story of how methotrexate made the leap from pediatric leukemia treatment to rheumatology’s most-used drug is itself revealing. Researchers noticed that weekly low doses produced anti-inflammatory effects that could not be fully explained by the cancer-killing mechanism.16PubMed. Landmark papers on the discovery of methotrexate for the treatment of rheumatoid arthritis and other systemic inflammatory rheumatic diseases The adenosine pathway, discovered later, explained most of what they were seeing. Today, methotrexate is considered the first-line disease-modifying drug for rheumatoid arthritis thanks to its combination of low cost, decades of safety data, and good long-term efficacy.17PubMed. Methotrexate: an old new drug in autoimmune disease
When your pharmacist hands you a bottle that says “immunosuppressant” on the label, that labeling is not wrong, but it reflects the drug’s full dose range. At the dose you are probably taking for an autoimmune condition, “immunomodulator” or “anti-inflammatory” would be a more accurate description of what it is doing most of the time.
Genetic Variation in How People Handle Methotrexate
Not everyone responds to methotrexate the same way, and genetics play a role. Researchers have studied variants in the MTHFR gene, which codes for an enzyme involved in folate metabolism, to see whether certain gene versions predict who will benefit from methotrexate and who will suffer more side effects.18Proceedings of the Bulgarian Academy of Sciences. Association of the MTHFR Gene Polymorphisms with Efficacy and Toxicity of Methotrexate in Patients with Juvenile Idiopathic Arthritis The idea makes intuitive sense: if your version of MTHFR already processes folate less efficiently, adding a drug that further disrupts folate pathways could tip you toward more side effects or altered drug response.
Methotrexate metabolism can also be complicated by kidney function, liver health, and interactions with other medications.19PubMed Central. Factors Underlying Failure of Methotrexate Treatment in Rheumatoid Arthritis: Implications in Personalized Care Some people reach effective drug levels easily on a low dose while others never seem to respond adequately. Personalized dosing based on genetics and other patient factors is an active area of research, though in everyday practice most doctors still adjust the dose based on how a patient responds clinically and what their blood work shows.
Pregnancy and Methotrexate
Whatever nuance exists about methotrexate’s immune effects, its reproductive toxicity is unambiguous. Methotrexate is a known teratogen in humans, meaning it causes birth defects.20Reproductive Toxicology. Fetal methotrexate syndrome: A systematic review of case reports Exposure during the first trimester can lead to a constellation of abnormalities known as fetal methotrexate syndrome, which can include skull, limb, and facial malformations. This risk applies at both high and low doses.
Women of childbearing age are typically advised to use reliable contraception while on methotrexate and to stop the drug well before attempting pregnancy, usually at least three months beforehand. Men on methotrexate have also been advised to stop before conception, though the evidence for paternal risk is less clear-cut. The drug is also sometimes used deliberately at higher doses to treat ectopic pregnancies, precisely because of its ability to halt rapidly dividing cells. If you are planning a pregnancy and currently take methotrexate, the conversation with your doctor about timing the switch to a safer medication is one of the most important you will have.
Screening Before Starting
Because methotrexate can reactivate latent infections by dampening immune surveillance, screening before starting the drug has become standard in rheumatology. A clinical audit of patients starting biologic or targeted synthetic disease-modifying drugs found that roughly one in five screened positive for latent tuberculosis.21PubMed Central. Clinical Audit of screening Latent TB, Hepatitis-C, and Occult Hepatitis-B in Rheumatoid arthritis’ patients starting biologic or targeted synthetic DMARDS Those patients received preventive treatment, and none developed active TB during therapy. Similar screening protocols for hepatitis B and C help identify people who need antiviral treatment before immunosuppressive therapy makes a dormant virus flare.
This kind of careful pre-treatment workup reflects the medical community’s pragmatic stance: methotrexate is treated as an immunosuppressant for safety purposes, even if its dominant mechanism is anti-inflammatory. The precautions err on the side of caution because the consequences of reactivating TB or hepatitis in someone whose immune system is even mildly suppressed can be severe. For the average patient whose screening comes back clear, these precautions remain invisible, just a blood draw or two before the first prescription is filled. But they are a concrete example of how the “immunosuppressant” label, even if it overstates the drug’s usual mechanism, shapes real-world clinical care in ways that protect patients.