Which of the following is an example of a pharmacodynamic drug-drug interaction?

• A. Coadministration of antithrombotic drugs leading to synergistic anticoagulation.
• B. Stimulation of hepatic metabolism decreasing drug exposure.
• C. Induction of renal excretion reducing drug levels.
• D. Competitive inhibition of P450 enzymes increasing drug clearance.

Answer: (C)

Pharmacodynamic interactions occur when two drugs affect the same physiological target or pathway, so their combined effect on the body is altered without necessarily changing how much of either drug is in the bloodstream. An example is using two antithrombotic drugs together—they both influence the coagulation system or platelet function, producing a greater anticoagulant effect (and bleeding risk) than either would alone. This is about the drugs’ effects on the body, not about changing drug levels.

The other scenarios are pharmacokinetic interactions, which change how much drug is in the body. Inducing hepatic metabolism or renal excretion shifts drug exposure by altering absorption, distribution, metabolism, or elimination, not by changing the drug’s effect at its target. Competitive inhibition of P450 enzymes also alters clearance/metabolism and thus exposure, again a pharmacokinetic effect.