In pharmacology, sometimes more than one drug is used to treat a disorder. The two or more drugs are used as they treat the disease more effectively than they treat the disorder independently. The two drugs interact in one of the two ways: the additive effect and the synergistic effect.
In the given question, the drugs when combined produce the sum of the drugs equal to the effect shown by the drugs individually therefore the effect of the drug is added.
This type of drug interaction is known as the additive effect.
