Allergic Drug Reactions Penicillin - probably the most common cause of allergic drug reactions   Drug-induced illness is one of the major public health problems today. In the US, between 3-5% of medical hospital admissions are due to adverse drug effects, and 10-20% of patients hospitalized for other reasons have any adverse response to a drug prescribed while they are hospitalized.   In clinical studies, hospitalized patients, who generally receive more than one drug during their stay, are the people most carefully monitored for side effects. About 30% of them develop some drug reaction during their hospitalization. Most of these are not allergic reactions, but many of the more serious ones are. Other studies attribute 2.7 to 34.4% of all drug reactions to an allergic response.   Not all allergic drug reactions are due to the drug itself. Some may be the response of the immune system to traces of impurities or additives in a medicine or to a drug’s metabolites, the substances that result from the chemical breakdown of the drug in the body.   Some drugs produce a single type of reaction; while others can trigger a number of effects. For example, an allergic reaction to penicillin may manifest itself in any one of at least seven forms, including anaphylaxis, fever, and severe contact dermatitis.   Certain parts of the body seem to be more susceptible to allergic drug reactions, particularly the skin, lungs, liver, kidneys, blood and blood vessels. A drug allergy may affect a single organ and a localized area of the body, or it may affect many organs and many sites the same time. Many allergic drug reactions were undetected Because drugs may affect several organs in the body, it is sometimes hard to decide whether a patient’s symptoms result from the illness itself or from an allergic reaction to a drug. For example, a rash may appear during or after some streptococcal infections. Yet, a rash may also be a symptom of an allergy to penicillin, an antibiotic used to treat the same streptococcal infections.   Common drugs that cause allergic reactions Penicillin is probably the most common cause of drug allergy, with some type of allergic response occurring in one of every 50 persons who take this antibiotic. About 76% of these responses are skin reactions (hives, rashes, contact dermatitis). About 22% are systemic (serum sickness, drug fever, vasculitis, angioedema, wheezing). Around 2% of the reactions to penicillin involve anaphylactic shock.   Other antibiotics that may cause drug allergy include: Neomycin used in eye drops Streptomycin Chloramphenicol used in treating Rocky Mountain spotted fever, typhoid fever, and certain types of meningitis Aspirin, which is used to relieve pain and reduce fever, may also result in drug allergy.   Other common drugs that may cause allergic reactions include: Insulin used for treating diabetes Anti-tuberculosis medications and other antibiotics not listed above Anticonvulsants, barbiturates, local anesthetics Organ extracts, vaccines, tranquilizers and sleeping pills, laxatives Drugs used to treat hyperthyroidism and heart disorders.