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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# JAK inhibitors

Small oral molecules that block Janus kinases inside cells, cutting the signal of many cytokines at once. Used in arthritis, colitis and some marrow diseases.

Updated 2026-11-01

## What they are

Tofacitinib and baricitinib are oral targeted DMARDs for rheumatoid arthritis and related diseases, usually after a TNF inhibitor has failed; ruxolitinib treats myelofibrosis, polycythaemia vera and graft-versus-host disease. Unlike biologics, they are tablets and act inside the cell.

## Mechanism of action

Many cytokine receptors (IL-2, IL-6, interferons, GM-CSF, erythropoietin and others) have no kinase of their own and depend on associated Janus kinases (JAK1, JAK2, JAK3, TYK2). When the cytokine binds, the JAKs phosphorylate STAT proteins, which move to the nucleus and switch on inflammatory genes. JAK inhibitors compete with ATP in the kinase site and stop this step.

Target: Janus kinases (JAK1, JAK2, JAK3, TYK2)

## Key points

- Boxed warnings for tofacitinib and baricitinib: serious infections, death, cancer (lymphoma, lung), major cardiovascular events and thrombosis, compared with TNF inhibitors in rheumatoid arthritis.
- Herpes zoster is particularly common.
- Labs: lymphocytes, neutrophils, haemoglobin, liver enzymes and lipids.
- Not combined with biologic DMARDs or potent immunosuppressants such as azathioprine or ciclosporin.
- Names: -citinib = JAK inhibitor.

## Drugs in this class

- [**Tofacitinib** An oral JAK inhibitor for rheumatoid arthritis, psoriatic arthritis, spondylitis and ulcerative colitis after a TNF… Janus kinases, mainly JAK1 and JAK3](https://durgical.com/en/pharmacology/tofacitinib/)
- [**Baricitinib** An oral JAK1/JAK2 inhibitor for rheumatoid arthritis after a TNF blocker, alopecia areata and severe COVID-19 in… Janus kinases JAK1 and JAK2](https://durgical.com/en/pharmacology/baricitinib/)
- [**Ruxolitinib** An oral JAK1/JAK2 inhibitor for myelofibrosis, polycythaemia vera and graft-versus-host disease. Blood counts and… Janus kinases JAK1 and JAK2](https://durgical.com/en/pharmacology/ruxolitinib/)

## Others in the class

- upadacitinib
- filgotinib

## Test yourself

Think of the answer, then tap the question to see it.

## Where do JAK inhibitors act, compared with biologic DMARDs?

Inside the cell, on the kinases attached to cytokine receptors; biologics act outside the cell on the cytokine or its receptor.

## Which viral infection is especially common with JAK inhibitors?

Herpes zoster (shingles).

## Which risks were higher with tofacitinib than with TNF inhibitors in rheumatoid arthritis?

All-cause mortality, major cardiovascular events, thrombosis, and lymphoma and lung cancer.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - TOFACITINIB tablet, film coated](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=10403307-e2f9-44d1-bc72-552fdba299d1) (DailyMed (U.S. National Library of Medicine), 2026); [Baricitinib: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a618033.html) (MedlinePlus (U.S. National Library of Medicine), 2022); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 39 (McGraw Hill, 2023).

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