When two drugs with similar pharmacological actions result in an effect equal to the sum of their individual effects, this is called?

The correct answer, additive effects, is based on the principle that when two drugs are administered together, and their pharmacological actions align, the resulting effect is the sum of their individual effects. This means that if one drug has an effect of X and the other has an effect of Y, when taken together, they will produce an effect of X + Y. This is commonly observed in medications where two drugs with similar mechanisms enhance the overall therapeutic outcome but do not enhance each other beyond their individual capabilities.

In contrast, synergism refers to a situation where two drugs work together in such a way that their combined effect is greater than the sum of their individual effects. Displacement relates to one drug altering the concentration of another drug in the bloodstream, typically through binding competition at the same receptor. Inhibition involves one drug reducing or blocking the effect of another. Each of these concepts differs fundamentally from additive effects, which specifically involves a straightforward cumulative action of two drugs.