A drug-drug interaction occurs when one drug interacts with or interferes with the action of another drug. For example, taking an antacid with oral tetracycline causes a decrease in the effectiveness of the tetracycline. The antacid chemically interacts with the tetracycline and impairs its absorption into the bloodstream, thus reducing the effectiveness of the tetracycline. Drugs known to cause interactions include oral anticoagulants, oral hypoglycemics, anti-infectives, antiarrhyth-mics, cardiac glycosides, and alcohol. Drug-drug interactions can produce effects that are additive, synergistic, or antagonistic.
ADDITIVE DRUG REACTION. An additive drug reaction occurs when the combined effect of two drugs is equal to the sum of each drug given alone. For example, taking the drug heparin with alcohol will increase bleeding. The equation one + one = two is sometimes used to illustrate the additive effect of drugs.
SYNERGISTIC DRUG REACTION. Drug synergism occurs when drugs interact with each other and produce an effect that is greater than the sum of their separate actions. The equation one + one = four may be used to illustrate synergism. An example of drug synergism is when a person takes both a hypnotic and alcohol. When alcohol is taken simultaneously or shortly before or after the hypnotic is taken, the action of the hypnotic increases. The individual experiences a drug effect that is greater than if either drug was taken alone. On occasion, the occurrence of a synergistic drug effect is serious and even fatal.
ANTAGONISTIC DRUG REACTION. An antagonistic drug reaction occurs when one drug interferes with the action of another, causing neutralization or a decrease in the effect of one drug. For example, protamine sulfate is a heparin antagonist. This means that the administration of protamine sulfate completely neutralizes the effects of heparin in the body.
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