Ketotifen

Ketotifen is a dual-action medication: it’s both a mast cell stabilizer and an H1 antihistamine. This combination makes it useful in MCAS because it both prevents Degranulation and blocks Histamine at H1 receptors.

How It Works

Mast cell stabilization: Inhibits calcium influx and downstream signaling required for degranulation. Similar conceptual mechanism to Cromolyn Sodium but through a different molecular pathway.

H1 antagonism: Competes with histamine at H1 receptors, blocking vasodilation, permeability, bronchoconstriction, and itch.

Unlike cromolyn, ketotifen is well-absorbed orally and reaches systemic circulation. This means it can stabilize mast cells throughout the body — not just in the gut. This is an advantage for patients with multi-system symptoms.

Side Effects

Ketotifen crosses the blood-brain barrier. The primary side effect is sedation, which can be significant initially. Most patients report that sedation improves after 1-2 weeks of consistent use as tolerance develops. Weight gain and increased appetite are also reported.

The sedation can be channeled by dosing at bedtime (see Sleep and Histamine). Some providers start with bedtime-only dosing and add daytime doses as tolerance allows.

Availability

Availability varies by country. In some regions it’s prescription-only oral; in others it’s available only as eye drops (Zaditor/ketotifen ophthalmic). Compounding pharmacies can prepare oral formulations where commercial oral products aren’t available.