JAK Inhibitor (Janus Kinase Inhibitor)

A JAK inhibitor blocks Janus kinase enzymes, which help pass inflammation signals inside immune cells. By blocking them, these drugs reduce the overactive immune response behind conditions such as rheumatoid arthritis, psoriatic arthritis, ulcerative colitis, and some skin diseases. Unlike many biologic drugs, JAK inhibitors are pills. Well-known brands include Xeljanz (tofacitinib), Olumiant (baricitinib), and Rinvoq (upadacitinib).

Their safety became a major issue after a large clinical trial required by the FDA compared Xeljanz with older TNF blocker drugs in rheumatoid arthritis patients. On September 1, 2021, the FDA announced it was requiring revisions to the boxed warning for Xeljanz, Olumiant, and Rinvoq to include the risks of serious heart-related events, cancer, blood clots, and death. The FDA also told doctors to reserve these drugs for patients who had not responded well to, or could not tolerate, one or more TNF blockers.

The warning led to lawsuits by patients who suffered heart attacks, strokes, blood clots, or cancers while taking these drugs, alleging the manufacturers should have warned sooner. These are individual injury claims. To pursue one, a person generally needs pharmacy records showing which JAK inhibitor they took and when, and medical records of the injury. Courts look closely at whether the injury happened while the person was on the drug and whether other risk factors explain it.

See the tofacitinib entry for more on Xeljanz, and our guides on class actions versus mass torts and how to sue a company.