Types of drug interactions

 1. Additive drug interaction: the effect of two substances interacting with each other corresponds to the sum of their individual effects o A + B = AB

o Example: aspirin coadministered with acetaminophen

2. Synergistic drug interaction: the effect produced by the interaction of two substances is greater than the sum of their individual actions o A + B > AB

o Example: aspirin coadministered with clopidogrel

3. Drug potentiation: the therapeutic effect of a substance is enhanced by another substance with no therapeutic action o A + 0 > A

o Examples: Carbidopa coadministered with levodopa (carbidopa blocks the peripheral conversion of levodopa)

o Cobicistat: used in antiretroviral therapy of HIV infection to decrease breakdown of antiretrovirals (e.g., darunavir, atazanavir)

4. Permissive drug interaction: the effect of a substance can only be achieved in the presence of another substance o A - B = 0

o Example: cortisol coadministered with catecholamine (cortisol increases the sensitivity of adrenoreceptors to catecholamines)

5. Antagonistic drug interaction: the effect produced by the interaction of two substances is smaller than the sum of their individual actions o A + B < AB

o Example: Ethanol is used to treat methanol toxicity (ethanol binds alcohol dehydrogenase with higher affinity than methanol and thereby competitively inhibits the formation of toxic metabolites)