Budesonide

Binds the glucocorticoid receptor about 200 times more strongly than cortisol, with weak mineralocorticoid activity. The receptor induces anti-inflammatory proteins such as lipocortins, which block phospholipase A2, and curbs NF-kB-driven inflammation. Swallowed budesonide is 85 to 95% broken down by CYP3A4 on its first pass through the liver, which limits whole-body exposure.

Like other inhaled corticosteroids: Inhaled corticosteroids enter airway cells and bind the glucocorticoid receptor, which moves to the nucleus and changes gene expression: anti-inflammatory proteins rise and cytokines and other mediators fall, so fewer mast cells, eosinophils and lymphocytes inflame the airway. Benefit builds over days to weeks, not at once. Inhaled, they act strongly where they land with little whole-body effect.

• Inhaled budesonide prevents attacks but does not relieve acute bronchospasm.
• Ketoconazole, ritonavir and grapefruit juice block CYP3A4 and raise systemic budesonide levels.
• Liver impairment raises exposure and the risk of steroid side effects.
• Moving from systemic steroids needs a slow taper to avoid adrenal insufficiency.

Good to know: Beyond inhalers it comes as capsules for Crohn's disease affecting the ileum or ascending colon; heavy first-pass breakdown limits how much reaches the rest of the body.

Easily mixed up with glucocorticoids, long-acting beta-2 agonists and leukotriene receptor antagonists.