BACKGROUND The use of opioids as analgesics is becoming increasingly widespread, which may have repercussions in patients with urticaria or asthma, as these agents frequently cause adverse reactions. METHODS We present three patients who developed allergic reactions after receiving codeine: two patients who developed acute urticaria, and a third asthmatic patient receiving specific immunotherapy who developed bronchospasm. Skin prick-testing (SPT) and intradermal reaction (IDR) tests with various opioids were performed, followed by controlled oral challenge. Prick tests and IDR were also carried out in 20 controls. RESULTS Similar SPT and IDR results were recorded in the three patients and in the controls. In the case of controlled oral challenge with codeine, patient 1 suffered bronchospasm, while patient 2 developed generalized urticaria. The test was not performed in the third patient. All of the patients tolerated tramadol 50 mg without problems. We advised the use of tramadol as analgesic and fentanyl or remifentanil as anesthetics. CONCLUSIONS In these types of manifestation, the pharmacological properties of the opioids used are highly important, particularly as regards their histamine-releasing potential. Codeine, morphine and pethidine present the greatest histamine-releasing capacity, while tramadol, fentanyl and remifentanil do not release histamine and their use is thus recommended in pulmonary disease requiring opioid administration. Cutaneous symptoms are more frequently caused by opioids than by respiratory symptoms, since these drugs act on the MTC mast cell population, which is more prevalent in the skin than in the lungs. Some of this action is inhibited by naloxone. CONCLUSIONS In most patients, these reactions are not IgE-mediated. Consequently, SPT and IDR are of little diagnostic value, and controlled oral challenging with the suspect drug or with one of the non-histamine releasing agents should be used. The patch test is useful in occupational contact dermatitis.