Adverse drug reactions

No drug is wholly nontoxic or completely safe. Adverse effects can range from minor reactions, such as dizziness or skin reactions, to serious and even fatal effects. Adverse reactions can be divided broadly into effects that result from an exaggeration of the basic action of the drug, which can usually be controlled by reducing the dosage, and effects that are unrelated to the basic action of the drug and occur in only a small proportion of individuals, irrespective of the dose given. Effects of the latter type are known as idiosyncratic effects and include some very severe reactions, such as sudden cardiovascular collapse or irreversible suppression of blood cell production. Many reactions of this type have an allergic basis. Toxic effects of this kind, though rare, are unpredictable and sometimes highly dangerous, and they severely limit the usefulness of many effective drugs. It has been increasingly recognized that drugs can produce other kinds of unwanted effects, such as interference with fetal development (teratogenesis) or long-term genetic damage that may make a person susceptible to the development of cancer.

The sporadic and delayed nature of many adverse drug reactions and the fact that they may not be predictable from animal tests poses serious practical problems. Often such effects are, and indeed can only be, discovered after a drug has been used in humans for some time.

An adverse drug reaction is defined by the World Health Organization as «one which is noxious and unintended, and which occurs at doses normally used in man for the prophylaxis, diagnosis, or therapy of disease, or for the modification of physiological function».

There are two main types of adverse drug reactions. Type A reactions are extensions of the drug's known pharmacology and are responsible for the majority of adverse drug reactions. They are usually dose-depensent and predictable, but can be due to concomitant disease states, drug-drug interactions, or food-drug unteractions. The ways to minimize type A reactions include an awareness of pharmacology of the drug prescribed, drug therapy monitoring and avoiding polypharmacy whenever possible.

Type B reactions include idiosyncratic reactions, immunologic or allergic reactions, and carcinogenic/teratogenic reactions. Type В reactions are usually not due to a known pharmacology of the drug, but seem to be a function of patient susceptibility. They are rarely predictable, and are usually not dose-dependent and seem to concentrate in certain body systems such as the liver, blood, skin, kidneys, the nervous system, and other body systems. Type В reactions are uncommon, but generally very serious and can be life-threatening. Except for immediate hypersensitivity reactions, type B reactions usually take 5 days before the patient demonstrates hypersensitivity to a drug. There is no maximum time for the occurrence of the reaction, but most occur within twelve weeks of therapy.

Not all drug interactions are clinically significant or cause an adverse effect. In some cases, interacting drugs can be prescribed as long as the patient is given proper instructions and is compliant. For example, cimetidine and an antacid can be prescribed to the patient, but they should be instructed not to take both medications at the same time. Multiple drug therapy including both prescription and OTC drugs can potentially lead to drug interaction. The more drugs used by the patient, the greater the potential for a drug interaction in the patient. Very often the patients consult different health care professionals who prescribe interacting medications. Elderly patients are at a higher risk of drug interaction than the younger ones. Older patients might have alterations in their physiological and physiopathological condition that lead to a change in body composition, GI motility, and drug absorption rate. Patients with predisposing conditions (diabetes, asthma, AIDS, and alcoholism), and patients who are clinically hypersensitive (atopic) are more at risk for drug interaction than nonatopic patients.

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