Additivity: when the effect of two drugs given in combination equals the mathematical summation of their effects when given alone.
Synergism: when the combine effect of two drugs is greater than the sum of their effects when given separately.
Potentiation: when one drug does not elicit a response on its own but enhances the response to another drug.
Examples of Drug-Drug Interactions: given responses to 4 drugs
- Drug A = 0 units response
- Drug B = 3 units response
- Drug C = 5 units response
- Drug D = 10 units response
Drug A + Drug B = 5 units >> potentiation
Drug B + Drug C = 8 unites >> additivity
Drug C + Drug D = 20 units >> synergism