Letters ta the Editor 545 generalized allergic contact dermatitis (3) or erythema nuiltiforme (4), have been described. This can occur if discontinuation ofthe use of nitroglycerin TTS is not carried out promptly (3).The diagnosis is based on the patch test. The concentrations and vehicles suggested in the literature for patchtesting are nitroglycerin 0.01% to 2% in petrolatum, 0.2-0.5% in water or ethanol. As nitroglycerin is difficult to handle because it is flammable, in some reported cases, nitroglycerin allergy has been proven by application of various eommercial preparations of nitroglycerin in different excipients (5). A positive reaction should lead to oral challenge testing in a hospital. Usually, oral and sublingual provocation do not produce any reactions, but a cross-sensitivity allergic reaction to nitroglycerin and isosorbide dinitrate has been described (6). REFERENCES 1. McKcnnn KE.