Potentiation occurs when two drugs are taken together and one of them intensifies the action of the other. This could be expressed by a +b= B. As an example, Phenergan(R), an antihistamine, when given with a painkilling narcotic such as Demerol(R) intensifies its effect, there by cutting down on the amount of the narcotic needed.
Synergism is similar to potentiation. If two drugs are taken together that are similar in action, such as barbiturates and alcohol, which are both depressants, an effect exaggerated out of proportion to that of each drug taken separately at the given dose may occur. This could be expressed by 1+1= 5. An example might be a person taking a dose of alcohol and a dose of a barbiturate. Normally, taken alone, neither substance would cause serious harm, but if taken together, the combination could cause coma or death.