Drug interactions

When two or several drugs are taken simultaneously by a patient, their effects can be modified.

Independent effects

The particular effects of two or several drugs can be independent one of the other while being complementary. For example, the action of an antibiotic can be independent of that of an analgesic prescribed simultaneously. The analgesic does not modify the antibiotic action and reciprocally.


There is drug-drug interaction when the effect of a drug is modified by another drug. So that this interaction appears, it is necessary for the 2 drugs to be simultaneously present in the body or that the effects of one of them still persist at the time of the administration of the second. The consequences of this interaction can be described as synergy, potentiation or antagonism.

  1. Synergy: the effects of 2 drugs are totally or partially additive.
  2. Potentiation: the effect of one drug is greatly increased by the intake of another drug itself without notable effect
  3. Antagonism: the effect of one drug is decreased or suppressed by another drug.

The interactions between two drugs depend on their direct and indirect effects and on modifications of their pharmacokinetic characteristics.

The interactions between drugs can be often foreseeable. Thus, the simultaneous administration of two drugs having similar effects (two anticoagulants, two vasoconstrictor agents, two hypnotics etc), acting by the same mechanism or not, have synergistic effects.

The pharmacokinetic interactions are often more unexpected. For example, many macrolides (erythromycin, troleandomycin), which are antibiotics, potentiate the effects of ergot derivatives such as ergotamine which is a vasoconstrictor agent. The explanation of this potentiation responsible of serious adverse effects is that macrolides inhibit the inactivation of ergotamine which accumulates in the body at toxic levels.

The interactions between drugs are observed in general when the drugs are taken simultaneously or with a short time interval, a few hours to one day. But interactions are possible with much longer intervals separating their intake, a few days to two weeks. In this last case, one at least of drugs has long lasting effects resulting for example from sustained-release or an irreversible enzyme inhibition necessitating a new synthesis of the enzyme.

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