Question

Warfarin drug molecules are considered highly protein-bound. What will happen if a patient on warfarin is also on phenytoin? Look up phenytoin to see about its affinity for protein. Then choose the correct answer.

a) No interaction will occur due to different protein-binding sites
b) Phenytoin's low protein-binding affinity will increase warfarin's effectiveness
c) A type of drug-drug interaction will occur in which competition for protein will result in an increase in free drug
d) Warfarin's protein-binding affinity will decrease due to phenytoin's presence

Answer 1

When warfarin and phenytoin, both of which have strong protein-binding affinities, are taken together, they compete for protein-binding sites, which can increase the free concentrations of these drugs leading to potential increases in their effects or toxicity.

Step-by-step explanation:

If a patient on warfarin, a medication that is highly protein-bound, is also prescribed phenytoin, which has a strong affinity for protein binding as well, a type of drug-drug interaction can occur. This competition for the protein-binding sites, primarily albumin, may result in an increase in the amount of free drug molecules of one or both medications. Typically, it is the unbound drug that is pharmacologically active, so if more warfarin becomes unbound due to the presence of phenytoin, the result could be an increased effect of the warfarin, potentially leading to bleeding complications. Similarly, an increase in free phenytoin could potentially lead to toxicity. Therefore, the correct answer is c) A type of drug-drug interaction will occur in which competition for protein will result in an increase in free drug.