Do you typically check vitamin B6 levels in patients on Sinemet?

I check B6 levels when a patient is not responding well to carbidopa/levodopa or if I have concerns about nutritional status. I have rarely found that correcting low or high B6 levels results in clinical change in patients.

If a patient were to seemingly lose efficacy from ongoing levodopa therapy, ...

The question may be a little more complex than it might seem at first glance.

While B6 is needed for dopamine synthesis, it does not simply diffuse into the cell for use, but is actively transported. As a result, blood (or general tissue) levels are only part of the story. Functional B6 deficiency (...