Nearly all of the pharmaceutical preparations that are used for anesthesia can release histamine. H1 and H2 receptor blockades are recommended for the following groups of patients at risk: 1) Patients who show hypersensitivity to i.v. administered medications and radiological contrast media. 2) Patients with risk factors in their case histories, e.g., atopy, previous cardiac and/or pulmonary damage, and an age of over 70 years. 3) Conditions with a pathological increase in the plasma histamine level. Such conditions include (a) polytrauma, chronic kidney insufficiency, patients under radium irradiation treatment; (b) the exploration phase during a surgical procedure, more frequent blood transfusions, extracorporeal circulation, and Palacos implantation. 4) Chymopapain for chemonucleolysis.