A variety of allergic or allergic-like NSAID hypersensitivity reactions follow the ingestion of NSAIDs. These hypersensitivity reactions differ from the other adverse reactions listed here which are toxicity reactions, . unwanted reactions that result from the pharmacological action of a drug, are dose-related, and can occur in any treated individual; hypersensitivity reactions are idiosyncratic reactions to a drug.  Some NSAID hypersensitivity reactions are truly allergic in origin: 1) repetitive IgE -mediated urticarial skin eruptions, angioedema , and anaphylaxis following immediately to hours after ingesting one structural type of NSAID but not after ingesting structurally unrelated NSAIDs; and 2) Comparatively mild to moderately severe T cell -mediated delayed onset (usually more than 24 hour), skin reactions such as maculopapular rash , fixed drug eruptions , photosensitivity reactions , delayed urticaria , and contact dermatitis ; or 3) far more severe and potentially life-threatening t-cell-mediated delayed systemic reactions such as the DRESS syndrome , acute generalized exanthematous pustulosis , the Stevens–Johnson syndrome , and toxic epidermal necrolysis . Other NSAID hypersensitivity reactions are allergy-like symptoms but do not involve true allergic mechanisms; rather, they appear due to the ability of NSAIDs to alter the metabolism of arachidonic acid in favor of forming metabolites that promote allergic symptoms. Afflicted individuals may be abnormally sensitive to these provocative metabolites or overproduce them and typically are susceptible to a wide range of structurally dissimilar NSAIDs, particularly those that inhibit COX1. Symptoms, which develop immediately to hours after ingesting any of various NSAIDs that inhibit COX-1, are: 1) exacerbations of asthmatic and rhinitis (see aspirin-induced asthma ) symptoms in individuals with a history of asthma or rhinitis and 2) exacerbation or first-time development of wheals or angioedema in individuals with or without a history of chronic urticarial lesions or angioedema. 
Risks associated with NSAIDs are detailed on the package inserts and Client Information Sheets that accompany all veterinary NSAIDS dispensed to clients. A Client Information Sheet should always be given to the client with each NSAID prescription. Pet owners should read this information carefully. Owners and veterinarians should carefully consider the potential benefits and risks of using an NSAID and other treatment options before deciding to use an NSAID. Use the lowest effective dose for the shortest duration consistent with individual response.
NSAIDs increase the risk of potentially fatal, stomach and intestinal adverse reactions (for example, bleeding, ulcers, and perforation of the stomach or intestines ). These events can occur at any time during treatment and without warning symptoms. Elderly patients are at greater risk for these adverse events. NSAIDs (except low dose aspirin) may increase the risk of potentially fatal heart attacks , stroke , and related conditions. This risk may increase with duration of use and in patients who have underlying risk factors for heart and blood vessel disease. Therefore, NSAIDs should not be used for the treatment of pain resulting from coronary artery bypass graft ( CABG ) surgery.