Drug interactions occur when a drug's mechanism of action is affected by the concomitant administration of substances such as foods, beverages, or other drugs. The cause is often inhibition of, or less effective action, of the specific receptors available to the drug. This influences drug molecules to bind to secondary targets, which may result in an array of unwanted side-effects.

The term selectivity describes a drug’s ability to target a single receptor, rendering a predictable physiological response. For example, the binding of acetylcholine to muscarinic tracheal smooth-muscle receptors (M₃) results in smooth muscle contractions.

When freely binding receptors interact with agonist- chemicals that activate receptors - and antagonists- that inhibit/ block activation - the opportunity for selective drugs to bind with the intended receptor cells decreases as most receptors are already accounted for. Therefore, the drugs are more likely to bind to other receptors relative to the intended receptor, causing different effects.

For example, consuming both Zolpidem (i.e., Ambien) and alcohol together, both which affect the GABA_A receptors, results in the overstimulation of this receptor, which can lead to loss of consciousness. The risk of a drug-drug interaction (DDI) increases with the number of drugs used. Over a third (36%) of the elderly in the U.S. regularly use five or more medications or supplements, and 15% are at risk of a significant drug-drug interaction.