🟥🟦🟥 A 65-year-old man with Parkinson’s disease has been on levodopa therapy for several months, resulting in significant improvement in his symptoms. However, he recently started taking an over-the-counter vitamin supplement, and his symptoms have worsened. Upon review, it is noted that the supplement contains high doses of pyridoxine (vitamin B6). Which of the following best explains this interaction?

A. Thiamine deficiency leads to reduced levodopa absorption.
B. Carbidopa competes with levodopa at the blood-brain barrier.  
C. Pyridoxine enhances peripheral decarboxylation of levodopa.  
D. Benserazide reduces the therapeutic action of levodopa.  
E. Pyridoxine reduces carbidopa’s effect on levodopa metabolism.  

Explanation  

A. Thiamine is not involved in levodopa metabolism or absorption; its deficiency is irrelevant in this context.
  
B. Carbidopa does not compete with levodopa but enhances its central action by inhibiting peripheral metabolism.  

C. Pyridoxine increases peripheral decarboxylation of levodopa, reducing its availability in the brain and worsening symptoms.  

D. Benserazide inhibits the peripheral metabolism of levodopa, improving its availability and action in the brain. 
 
E. Pyridoxine does not reduce carbidopa’s inhibitory effect but directly promotes peripheral levodopa metabolism.  

Keynote  

High doses of pyridoxine (vitamin B6) can reduce the therapeutic effect of levodopa by promoting its peripheral decarboxylation, decreasing the amount available for the brain. This interaction can lead to the worsening of Parkinson’s symptoms, highlighting the need to avoid pyridoxine supplementation in such patients unless combined with a decarboxylase inhibitor like carbidopa or benserazide.