Methotrexate

Methotrexate is prescribed to treat:

• Rheumatoid arthritis, psoriasis, and certain cancers • Various related conditions in the immunology category • Associated symptoms and complications

It is an important medication that helps manage these conditions effectively.

Methotrexate is a disease-modifying antirheumatic drug and the anchor of treatment for rheumatoid arthritis, psoriatic arthritis, and psoriasis. At the far higher doses used in oncology it is a chemotherapeutic agent, and the gulf between those two dose ranges is the source of the most dangerous medication errors associated with any drug in common outpatient use.

Mechanism of Action

Methotrexate is a folate analogue that competitively inhibits dihydrofolate reductase, blocking conversion of dihydrofolate to tetrahydrofolate and therefore impairing synthesis of purines, thymidylate, and ultimately DNA. That antiproliferative effect explains its oncologic use and its toxicity to rapidly dividing tissues — bone marrow, gastrointestinal mucosa, and hair follicles.

The anti-inflammatory mechanism at low weekly doses is different and only partly overlapping. It is attributed largely to inhibition of AICAR transformylase, which increases extracellular adenosine — a potent endogenous anti-inflammatory mediator — along with effects on lymphocyte proliferation and cytokine production. This is why low-dose methotrexate works in inflammatory arthritis without producing meaningful immunosuppression of the kind that follows cytotoxic dosing.

Folate supplementation is standard alongside low-dose therapy and is the single most useful tolerability intervention. It substantially reduces mucositis, nausea, and transaminase elevation without meaningfully reducing efficacy, and a patient struggling with side effects who is not taking folate should be asked about it before the methotrexate dose is reduced.

The Weekly Dosing Rule

Low-dose methotrexate is dosed once weekly, not daily, and daily administration of a weekly dose causes severe and sometimes fatal toxicity — bone marrow failure, mucositis, and hepatic and renal injury. This error has occurred often enough to prompt repeated safety alerts from regulators and pharmacy bodies worldwide.

The safeguards are practical. The dosing day should be named explicitly and written down rather than described as weekly. Patients should be able to state which day they take it. Dispensing quantities should match a weekly schedule. Any new clinician, pharmacist, or hospital admission should have the weekly frequency confirmed rather than inferred, since transcription into a daily medication list is exactly where the error occurs. Patients should be told directly, and without softening it, that taking it daily can be fatal — this is one of the few instances where alarming a patient is the safer choice.

Clinical Use

In rheumatoid arthritis, methotrexate is the first-line DMARD and remains the drug against which others are compared. It is also the usual backbone for combination therapy: biologics such as adalimumab and etanercept are typically added to methotrexate rather than substituted for it, and the combination outperforms either alone.

It is used in psoriasis and psoriatic arthritis, in some presentations of lupus, and in inflammatory bowel disease. It is frequently the agent that allows prednisone to be tapered off in steroid-dependent inflammatory disease, which is one of its most valuable roles.

Onset is slow — six to twelve weeks before full effect — so bridging with a corticosteroid or NSAID is common while waiting. Patients told to expect improvement within days conclude it has failed and stop. Subcutaneous administration achieves more reliable absorption than oral at higher doses and often resolves both gastrointestinal intolerance and apparent non-response. Our musculoskeletal and immunology teams manage these regimens, and the American College of Rheumatology publishes patient-level guidance.

Monitoring and Follow-Up

Methotrexate requires ongoing laboratory monitoring and this is not optional. Complete blood count, liver enzymes, and creatinine are checked at baseline, frequently during initiation and after dose increases, and at regular intervals thereafter once stable. The specific schedule follows rheumatology guidance and the patient's risk profile.

Bone marrow suppression, hepatotoxicity, and nephrotoxicity are the three effects being watched for. Kidney function matters doubly, because methotrexate is renally cleared and declining function causes accumulation and therefore toxicity — a particular hazard in older patients whose creatinine looks acceptable while eGFR has fallen.

Pulmonary toxicity is rare but serious, presenting as a dry cough and progressive breathlessness that can develop at any point during therapy. New respiratory symptoms in a patient on methotrexate warrant evaluation rather than being attributed to infection by default.

Alcohol raises hepatotoxicity risk and intake should be discussed explicitly. NSAIDs, trimethoprim-sulfamethoxazole, and proton pump inhibitors all interact — trimethoprim-sulfamethoxazole is particularly dangerous because it adds its own antifolate effect and can precipitate profound marrow suppression. Live vaccines are avoided.

Folate deserves more emphasis than it usually receives, because it is the difference between tolerating methotrexate and abandoning it. Standard practice is folic acid supplementation on the non-methotrexate days, and patients who stop the folate because it seems like an optional vitamin frequently return with mouth ulcers, nausea, and raised transaminases that are then attributed to the methotrexate dose itself. Confirming the folate is actually being taken should precede any dose reduction for tolerability. The MedlinePlus methotrexate entry sets out the prescribing detail, including the supplementation schedule.

Special Populations

Methotrexate is a known teratogen and is contraindicated in pregnancy. Effective contraception is required for both women and men during treatment and for a defined period afterwards, and this conversation must happen before the first dose rather than at some later review. It is contraindicated in breastfeeding.

In older adults, reduced kidney function and polypharmacy both increase risk, and doses are lower with closer monitoring. Significant kidney impairment, liver disease, heavy alcohol use, and pre-existing marrow suppression are contraindications. Patients should be screened for hepatitis B and C and for latent tuberculosis before starting, particularly where a biologic will follow.

When to Contact Your Doctor

Seek urgent care for fever, sore throat, or any sign of infection, which may indicate marrow suppression; for mouth ulcers or severe mouth soreness, which is often the earliest sign of toxicity; and for new cough or breathlessness. Report unusual bruising or bleeding, extreme fatigue, yellowing of the skin or eyes, dark urine, or persistent nausea with abdominal pain.

Confirm your dosing day if there is ever any uncertainty, and never take a missed dose the following day without asking. Tell every clinician and pharmacist that you take methotrexate weekly, and check before starting any new medication — particularly antibiotics.

To review your monitoring schedule, discuss side effects, or plan a change in therapy, contact us or schedule a visit.