Mast-Cell Stabilizers

The allergic response begins when an allergen cross-links IgE on a mast cell, causing it to degranulate — dumping histamine and other inflammatory mediators that drive bronchoconstriction, mucus, and swelling. Antihistamines block histamine after it is released; mast-cell stabilizers act one step earlier — they prevent the mast cell from degranulating in the first place, so the mediators are never released.

This makes them purely preventive. Cromolyn must be taken regularly, before exposure (or before exercise), and it takes days to weeks to reach full effect. The corollary is a key teaching point: cromolyn does NOTHING for an acute attack — it cannot reverse bronchospasm that is already happening. A rescue bronchodilator is still required for symptoms.

Because cromolyn is barely absorbed systemically and works locally at the mucosa, it is a very safe respiratory drug — its main adverse effects are minor local irritation (throat irritation, cough, bad taste, or nasal stinging with the spray). That safety supports its use in children (2 years and older) and for allergic rhinitis (Nasalcrom) and allergic conjunctivitis as well as asthma. In pregnancy, animal reproduction studies showed no fetal malformations, but there are no adequate studies in pregnant women, so the labels advise use only if clearly needed.

The trade-off is modest potency: it is a second-line controller, generally less effective than inhaled corticosteroids, and its need for frequent, consistent dosing challenges adherence. It is used as add-on prophylaxis — for example, taken before a known trigger or exercise.

Nedocromil, the class’s other drug, is no longer marketed in the US — not the Tilade inhaled products, the Alocril eye drops, or the generic eye drops. The maker of the Tilade inhaler stopped producing it in July 2008, and because the inhaler used a CFC (ozone-depleting) propellant, an FDA rule barred CFC nedocromil inhalers from the market after June 14, 2010; the FDA withdrew the inhaler’s approval at the holder’s request in 2017. For Alocril, the FDA determined in 2024 that it was not withdrawn for reasons of safety or effectiveness; the notice gives no reason for the discontinuation. The FDA expected former Tilade users to be well served by inhaled corticosteroids, naming montelukast and nebulized cromolyn as other options. Clinical references still describe nedocromil — for example, among the mast-cell stabilizers used for long-term control of allergic conjunctivitis, including vernal keratoconjunctivitis — so learners will meet the name even though no nedocromil product is marketed in the US.