Food and Drug Administration (FDA): Drug Interaction
Drug Digest (Check Interactions):
For more information on how drugs interact or to look-up how specific drugs will interaction with each other.
Discover your Health (Health Discovery):
For general information regarding health and other health related issues.
A licensed drug taken to cure or reduce symptoms of an illness or medical condition.
A kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect. Combinations of many simple interactions can lead to surprising emergent phenomena.
A situation in which one or more substances affect the activity or effect of a drug, (i.e. the effects are increased or decreased, or they produce a new effect that neither produces on its own). Interactions may exist between drugs &drugs (drug-drug interaction) drugs & foods (drug-food interactions), as well as drugs & herbs (drug-herb interactions). Generally speaking, drug interactions are best avoided, due to the possibility of poor or unexpected outcomes.
In some instances, drug interactions have been deliberately used to great effect, such as the co-administration of probenecid with penicillin prior to mass production of penicillin. Because penicillin was difficult to manufacture, it was worthwhile to find a way to reduce the amount of penicillin required for a course of therapy. Since probenecid reduces the excretion of penicillin from the body, a dose of penicillin will stay around the body for a longer period of time when taken with probenecid. Hence, probenecid allows individuals to take less penicillin over a course of therapy. In the present day, probenecid is generally no longer used for this purpose, since penicillin is now produced on a large-scale basis.
Used as an advantage is the co-administration of carbidopa with levodopa (available as Carbidopa/levodopa). Levodopa is used in the management of Parkinson's disease and must reach the brain in an un-metabolized state to be beneficial. When given by itself, levodopa is metabolized in the peripheral tissues outside the brain, which decreases the effectiveness of the drug and increases the risk of adverse effects. However, since carbidopa inhibits the peripheral metabolism of levodopa, the co-administration of carbidopa with levodopa allows more levodopa to reach the brain un-metabolized and also reduces the risk of side effects.
Drug interactions may be the result of various processes. These processes may include alterations in the pharmacokinetics of the drug, such as alterations in the Absorption, Distribution, Metabolism, and Excretion (ADME) of a drug. Alternatively, drug interactions may be the result of the pharmacodynamic properties of the drug (e.g. the co-administration of a receptor antagonist and an agonist for the same receptor).