Inactive ingredients are supposed to improve a medication’s taste, appearance, absorption, or shelf life without any noticeable effect. But they aren’t always behind-the-scenes players. Some are allergens that can cause adverse events, and they’re present in nearly all prescription and over-the-counter pills and capsules, according to a recent analysis in Science Translational Medicine.

The analysis indicates that clinical reports of adverse reactions triggered by inactive ingredients are on the upswing. Some are type 1 hypersensitivity allergic reactions that produce immunoglobulin E, causing symptoms such as urticaria, angioedema, bronchospasm, or anaphylaxis.