Two drugs can have a synergistic effect if they increase each other's effectiveness when taken together. There are two main types of synergistic drug interactions. Additive synergistic reactions are those in which the combined effects of two or more drugs are equal to the sum of those drugs' individual effects. Enhanced synergy occurs when the combined effect of the drugs is greater than the sum of those drugs' individual effects.
Sometimes, synergistic drug interactions are desired, and doctors take advantage of them in order to yield benefits for the patient. Other times, these drug interactions are unexpected and unwanted. In general, the more drugs a patient takes, the greater his or her chance of experiencing an interaction. Ailments such as liver malfunction and hormonal imbalances may cause a patient to experience a synergistic drug interaction that does not typically occur in a healthy patient.
The opposite of a synergistic effect is an antagonistic effect, which occurs when one drug cancels out another drug's effects. Antagonistic drug interactions occur when taking one drug partially or completely negates the effect of another drug a patient is taking. Doctors may use certain antagonist drugs to their advantage. For example, a patient taking an antibiotic drug that is causing nausea may also be prescribed a drug that counteracts the nausea-inducing effects of the first drug.