Drug interactions occur when a drug's mechanism of action is disrupted 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 begin binding to other secondary receptors, causing side effects. For example, consuming both Zolpidem (i.e., Ambien) and alcohol (both depressants) influxes the GABAA receptors, resulting in the over-stimulation of sleep-inducing chemicals, resulting in unconsciousness. 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. Based on pharmacodynamics Drug interactions can be additive (the result is what you expect when you add together the effect of each drug taken independently), synergistic (combining the drugs leads to a larger effect than expected), or antagonistic (combining the drugs leads to a smaller effect than expected). On some occasions, it is difficult to distinguish between synergistic or additive interactions, since the individual effects of each drug may vary from patient to patient. A synergistic interaction may be beneficial for patients, but may also increase the risk of overdose. Drug interaction predictors enable risk assessment of multiple drugs simultaneously with visualizations of risk per therapeutic class, to indicate a spectrum from no risk to high risk. Both synergy and antagonism can occur during different phases of the interaction between a drug and an organism. The different responses of a drug receptor have resulted in several classifications, such as partial agonist, competitive agonist, an inverse agonist. These concepts have fundamental applications in the pharmacodynamics of these interactions. The proliferation of existing classifications at this level and lack of knowledge around drug mechanisms means that it is difficult to