Many of the drugs in the same class exert additive effect as they have a similar therapeutic mechanism of action. For example, the calcium carbonate, magnesium, and aluminium salts are all antacids with the mechanism of using the negative ion to neutralize the acid in the stomach. The antacids have no interaction between them, so they would be considered to have additive effect when taken together.
Drugs that are in the same class, but do not have the same target, may also act additively by interacting with different targets in the same pathway. For example, propofol and sevoflurane can both produce anesthetic effects. Propofol can potentiate the activity of GABA receptor and act on α, β and γ subunits, while sevoflurane enhances the response of the GABA receptor to endogenous GABA by binding to the α1-subunit. By using Dixon up-down method, a trial has shown that the effect in producing anesthetic effects between propofol and sevoflurane is additive.