Adverse drug reactions occur in 10-20% of hospitalized patients, and up to one-third of these are of an allergic or pseudo-allergic nature. Allergic reactions are unpredictable adverse effects that are linked to immunological mechanisms. Pseudo-allergic reactions are unpredictable adverse reactions that are clinically similar to allergic reactions, but for which there are no drug-specific antibodies or T lymphocytes. There is a paucity of tools that allow a definite diagnosis, and most of the available ones still require validation. Therefore, there are few true epidemiological data, and most of those that are available, including incidence, mortality and socioeconomic impact data, should be interpreted with caution.