Drug synergism, Potentiation and Additive Effects

Drug synergism

Drug synergism: This is an interaction between two or more drugs that causes the total effect of the drugs to be greater than the sum of the individual effects of each drug.For example, both sulphamethoxazole and trimethoprim are antibiotics, but together they produce a greater inhibition of organisms than expected from the sum of their individual effects. This is true for most antibiotics in different classes and it is a beneficial effect of drug synergism.
Additive: This is a situation where a combination of the effects of two drugs equals the sum of each drug given alone. For instance, the combination of benzodiazepines and alcohol as CNS depressants.
Potentiation: This is a situation that occurs when one drug does not elicit a response on its own but enhances the response to another drug. This involves the increased effect of a drug when combined simultaneously with another that has no effect. An example is a probenecid (a non antibiotic) giving together with penicillin to inhibit its secretion in the kidneys and thus enhancing the action of penicillin in the body. This is a beneficial effect of potentiation. Isopropanonol alone, for example is not hepatotoxic; however, it greatly increase the hepatotoxicity of carbon tetrachloride. This is a harmful effect of potentiation.

Drug A = 0 units response
Drug B = 5 units response
Drug C = 10 units response
Drug D = 15 units response