A<unk>  is a situation in which a substance affects the activity of a<unk> , i.e. the effects are increased or decreased, or they produce a new effect that neither produces on its own.  Typically,<unk>  between<unk> s come to mind <unk> -drug<unk> .  However,<unk> s may also exist between<unk> s & foods <unk> -food<unk> s), as well as<unk> s & herbs <unk> -herb<unk> s).
Generally speaking,<unk> s are best avoided, due to the possibility of poor or unexpected outcomes. However, in some instances,<unk> s 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.
A contemporary example of a<unk>  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<unk>  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.<unk> s may be the result of various processes.  These processes may include alterations in the pharmacokinetics of the<unk> , such as alterations in the Absorption, Distribution, Metabolism, and Excretion (ADME) of a<unk> .  Alternatively,<unk> s may be the result of the pharmacodynamic properties of the<unk> , e.g. the co-administration of a receptor antagonist and an agonist for the same receptor.
Drug interaction
Opportunistic infection
Anaerobic infection
Adverse drug reaction
Infection
Nosocomial infection
Medication
Vascular injury