A patient is receiving warfarin (Coumadin), which is 98% bound to plasma proteins. The antidepressant paroxetine (Paxil), which is 95% bound to plasma proteins, is added to the patient's daily medication. If the paroxetine displaces warfarin from its protein binding sites, which of the following will most likely to occur?

The correct answer is D: Increased effect, and toxicity from warfarin. Paroxetine, by displacing warfarin from plasma proteins, can increase the concentration of free warfarin in the bloodstream. Since only the free (unbound) fraction of warfarin is pharmacologically active, this can lead to an increased effect of warfarin, potentially causing excessive anticoagulation and increased risk of bleeding, which is considered as toxicity. The other choices are incorrect because: A) Diminished effect from warfarin would not occur as the active free fraction of warfarin would increase. B) Diminished effect from paroxetine is unlikely as paroxetine's effect is not directly related to its protein binding. C) Increased effect and toxicity from paroxetine is less likely as paroxetine's displacement of warfarin does not increase the concentration of free paroxetine significantly.