text: Additive interaction or additive effect can be found in the treatment of the majority of common diseases. The combination of drugs with different effects has the benefit of using each drug at its optimal dose. This decreases the possibility of using a higher dose of a single medication if the previous dose is ineffective in treating diseases or relieving symptoms. The significance of using drugs with optimal dose is lowering the occurrence of intolerable side effects, adverse reactions, and possible drug toxicity in patient's body. This increases the safe use of drugs and increases patient compliance with the therapy.
  One of the examples is the use of calcium channel blocker and beta-blocker. They are drugs that can be used to treat stable angina. They can both decrease the frequency of angina, aiming to relieve the symptoms of angina. There are controlled, double blind clinical trials and studies involving patients with preserved left ventricular function demonstrating that the combination of calcium channel blocker and beta blocker has an additive cardio depressant effects when comparing with either drug class alone. The combination therapy is used when a single medication fails to produce a therapeutic effect. Choosing the optimal dose of the two medications in the combination therapy prevents the use of an extreme high dose of a single medication alone, leading to adverse effects.