Immediate hypersensitivity reactions to nonsteroidal anti-inflammatory drugs (NSAID) can be classified into allergic (immune-mediated) reactions to a single NSAID molecule or a group of chemically similar structure, and pseudo-allergic (or intolerance) linked to pharmacological Cox-1 inhibition. Distinction between these two entities is clinically difficult but important to quantify risks associated with the prescription of another NSAID. Oral challenge test can help the diagnosis. In case of an allergic reaction, a structurally chemically different NSAID will be offered. In case of a pseudo-allergic reaction, cross-sensitivity between non-selective NSAID is common; in these patients selective inhibitors of Cox-2 may be considered. If necessary, tolerance to acetylsalicylic acid can be induced, but has to be maintained by continous administration of the drug.