Which vitamin has been shown to reverse the effects of levodopa?

• A. Vitamin B12
• B. Vitamin C
• C. Vitamin B6
• D. Vitamin K

Answer: (C)

Vitamins can influence levodopa therapy by affecting how levodopa is converted to dopamine outside the brain. Levodopa must reach the CNS to be effective, but if it’s turned into dopamine in the periphery first, less reaches the brain. Vitamin B6 acts as a cofactor for the enzyme that converts levodopa to dopamine in the periphery, increasing this peripheral conversion and reducing the amount that enters the brain. That can reverse the therapeutic effect of levodopa unless a peripheral decarboxylase inhibitor like carbidopa is also present, which blocks that peripheral conversion. The other vitamins listed do not participate in this metabolic step, so they don’t reverse levodopa’s effects.