Methotrexate slows down your immune system and reduces inflammation by interfering with how certain cells use folate, a B vitamin essential for cell growth and division. At the low doses used for autoimmune conditions like rheumatoid arthritis and psoriasis (typically 5 to 20 mg once a week), it calms an overactive immune response. At the much higher doses used in cancer treatment, it kills rapidly dividing cells outright. The dose determines whether methotrexate acts more like an immune modulator or a chemotherapy drug, but the basic mechanism is the same.
How It Works Inside Your Cells
Your cells need folate to build DNA and replicate. Methotrexate blocks an enzyme called dihydrofolate reductase, which converts folate into its active form. Without that active form, cells struggle to produce the building blocks of DNA. This hits fast-growing cells hardest, including the immune cells driving inflammation in autoimmune disease.
The effects go deeper than just slowing cell division. Methotrexate also triggers the release of adenosine, a molecule that acts as a natural brake on inflammation. It reduces levels of ATP and GTP, two energy molecules that T cells (a type of white blood cell) need to multiply. The combined result is fewer inflammatory immune cells and less of the damage they cause to joints, skin, and other tissues.
What It Does to Your Immune System
In autoimmune conditions, your immune system attacks your own tissues. Methotrexate targets this process at several points. It suppresses T cells, which are the primary drivers of inflammation in rheumatoid arthritis, and reduces their production of TNF-alpha, a signaling protein that amplifies inflammation throughout the body. It also dials down other inflammatory signals like IL-1, IL-6, and interferon-gamma, while leaving anti-inflammatory signals relatively untouched.
This selective dampening is why methotrexate works so well for autoimmune disease without completely wiping out your immune defenses. You’re still protected against most infections, though your ability to fight them is somewhat reduced. That trade-off is why doctors monitor your blood counts regularly while you’re on the drug.
Common Side Effects
The most frequent complaints are nausea, fatigue, and mouth sores. These happen because methotrexate affects all fast-dividing cells, not just immune cells, and the lining of your mouth and digestive tract turns over quickly. Some people also experience headaches, hair thinning, diarrhea, and loss of appetite. Nausea tends to be worst in the day or two after your weekly dose, then fades.
Taking folic acid supplements significantly reduces these side effects. A common recommendation is 5 mg of folic acid the morning after your methotrexate dose, though your prescriber may suggest a different schedule. Folic acid replaces what methotrexate depletes in healthy cells without undermining the drug’s effect on your immune system.
If oral methotrexate causes significant stomach problems, a subcutaneous injection (a small shot under the skin you can give yourself) delivers the drug with better absorption and often fewer gastrointestinal side effects. Studies from Johns Hopkins Arthritis Center show the injectable form is actually more effective than oral tablets, with no increase in adverse events.
Effects on Your Liver
Your liver processes methotrexate, and the drug can cause elevated liver enzymes, a sign of liver stress. In most people on low-dose therapy, this is mild and reversible. Over years of use, though, there’s a small risk of liver fibrosis or scarring, especially if you have other risk factors like obesity, type 2 diabetes, or heavy alcohol use.
This is why you’ll have regular blood tests while taking methotrexate. These check your liver enzymes along with your blood cell counts and kidney function. Most rheumatologists order these tests every few months once you’re on a stable dose. The monitoring is routine and catches problems early, well before any lasting damage occurs.
Lung Complications
Between 1% and 7% of people on methotrexate develop some form of lung irritation. The most concerning is methotrexate-induced pneumonitis, an inflammatory reaction in the lungs that typically shows up within the first year of treatment, though it’s been documented anywhere from four months to 11 years after starting the drug. Symptoms include a persistent dry cough, shortness of breath, and sometimes fever. These overlap with common illnesses, which can make the problem easy to miss. If you develop an unexplained cough or increasing breathlessness while on methotrexate, it’s worth flagging promptly.
Alcohol and Methotrexate
The standard advice has shifted over the years. The American College of Rheumatology’s original 1994 guidelines recommended near-total abstinence, noting there was simply no data on what amount of alcohol was safe. More recent research has painted a less restrictive picture. The largest study on the topic found no increase in liver enzyme elevations among patients who drank fewer than 14 units of alcohol per week (roughly 6 to 7 standard drinks) while on methotrexate. British guidelines now advise staying well within national drinking recommendations rather than avoiding alcohol entirely.
That said, both alcohol and methotrexate stress your liver independently. If you have other liver risk factors, even moderate drinking may be worth reconsidering.
Pregnancy and Fertility
Methotrexate can cause serious birth defects and pregnancy loss. It is never taken during pregnancy. If you’re planning to conceive, the drug label recommends stopping methotrexate 3 to 6 months before trying, though some providers consider 1 to 3 months sufficient to clear the drug from your body. This applies to both women and men. Men should use effective contraception while on methotrexate and for at least 3 months after their final dose.
Low-Dose vs. High-Dose Treatment
The experience of taking methotrexate for an autoimmune condition is vastly different from receiving it as cancer chemotherapy. For rheumatoid arthritis or psoriasis, the typical dose is 5 to 20 mg taken once a week, often as a single oral tablet or injection. At this level, the drug modulates your immune system rather than destroying cells wholesale. Side effects are real but generally manageable, and many people take methotrexate for years.
In oncology, methotrexate is given at doses hundreds of times higher, often intravenously, over short treatment cycles. The goal shifts from immune suppression to killing cancer cells directly by arresting them mid-division. The side effect profile at these doses is far more intense and requires close hospital monitoring. If you’ve been prescribed low-dose methotrexate for an autoimmune condition and find alarming information online, check whether it refers to chemotherapy dosing, because the two situations are not comparable.