How Does Methotrexate Work? Mechanism of Action Explained in Plain English

Methotrexate is one of the most studied drugs in medicine. Its story begins with folic acid — a vitamin essential to cell division — and how blocking it can kill cancer cells or calm an overactive immune system. Here's how it works, in plain language.

Why Folic Acid Matters to Cells

Every time a cell divides, it needs to replicate its DNA — a process that requires a continuous supply of DNA building blocks called nucleotides. The production of these building blocks depends on folate (folic acid), specifically its active form: tetrahydrofolate (THF). Think of THF as the enzyme cofactor that enables cells to build new DNA.

Without adequate THF, cells cannot replicate their DNA and therefore cannot divide. This is exactly where methotrexate intervenes.

How Methotrexate Blocks Folate Metabolism

Methotrexate looks almost identical to folic acid at a molecular level. Because of this structural resemblance, it is taken up by cells using the same transport mechanism (the reduced folate carrier, SLC19A1) that normally imports folate.

Once inside the cell, methotrexate inhibits a critical enzyme called dihydrofolate reductase (DHFR). This enzyme's job is to convert dihydrofolate into tetrahydrofolate. By blocking DHFR, methotrexate cuts off the cell's supply of active folate — essentially starving the cell of the material it needs to build DNA.

The result: DNA synthesis stops. The cell cannot divide. This is the primary mechanism by which methotrexate kills cancer cells — it targets rapidly dividing cells, which include malignant cells but also bone marrow cells, gut lining cells, and hair follicle cells (explaining some of the side effects).

Why Does It Also Work for Arthritis and Psoriasis?

Here's where methotrexate's pharmacology gets more nuanced. At the low weekly doses used for rheumatoid arthritis and psoriasis, methotrexate does not primarily work through DNA synthesis inhibition — the cells aren't dividing fast enough for this to explain its effect. Instead, its anti-inflammatory mechanism operates through a different pathway.

At low doses, methotrexate interferes with folate-dependent reactions that produce a compound called AICAR. When AICAR accumulates inside cells, it triggers the release of adenosine — a natural anti-inflammatory molecule that acts on adenosine receptors throughout the immune system.

Adenosine has multiple immunosuppressive effects:

Reduces proliferation of T lymphocytes (key drivers of RA joint inflammation)

Decreases production of inflammatory cytokines (signaling proteins that fuel inflammation)

Inhibits synthesis of immunoglobulins and rheumatoid factor — proteins that drive autoimmune damage in RA joints

This is why methotrexate is called a "pleiotropic" drug — it has multiple mechanisms, acting differently in different contexts and at different doses.

Why Folic Acid Is Given Alongside Methotrexate

At low doses, the DNA-synthesis-blocking mechanism doesn't fully explain methotrexate's efficacy in RA — but it does explain some of its side effects. Normal cells that divide rapidly (gut lining, bone marrow, hair follicles, oral mucosa) are also affected. Supplemental folic acid "rescues" these normal cells by providing enough folate to maintain basic cell function, without undoing methotrexate's anti-inflammatory effect.

At high chemotherapy doses, leucovorin (folinic acid) — an active form of folate that bypasses DHFR — is used for rescue therapy to protect normal cells after high-dose methotrexate administration in cancer protocols.

How Is Methotrexate Eliminated From the Body?

Methotrexate is primarily eliminated by the kidneys (renal excretion). This is why kidney function must be monitored, and why NSAIDs — which reduce kidney blood flow — can dangerously increase methotrexate levels by reducing its excretion. The elimination half-life is approximately 3–10 hours for standard doses, though active polyglutamate forms can remain in tissues much longer.

For more detail on how these mechanisms translate to real-world safety considerations, read our posts on methotrexate drug interactions and methotrexate side effects.