An antagonistic drug interaction is one drug limiting the action another (1 + 1 = 1). An additive drug interaction is two drugs producing the intended effect (1 + 1 = 2). A synergistic drug reaction is one drug potentiating or increasing the effect of the other (and possibly vice versa; 1 + 1 = 3). An idiosyncratic drug reaction (not a drug interaction) is an unusual and unexpected adverse event caused by the drug.