Vitamin B6. Levodopa is used to treat Parkinson’s disease because it can cross the blood-brain barrier and affect nerve cells inside the brain. Once there, it is converted into dopamine, which helps reverse the effects of Parkinson’s disease, which is thought to be caused by a degeneration of the cells that produce dopamine.
Vitamin B6 breaks down levodopa and removes it from the body, so taking a vitamin B6 supplement with levodopa is likely to reduce its effectiveness. However, modern levodopa is often paired with carbidopa in the same pill. Carbidopa inhibits the process by which vitamin B6 breaks down levodopa, so indirectly carbidopa helps increase the long-term concentration of levodopa in the bloodstream, and thus increases its effectiveness in the brain.