Based on in vtro results and a n=1 case study:
"Levodopa and carbidopa are reported to be degraded by magnesium oxide (MgO), which is often used as a laxative for patients with Parkinson's disease (PD). Ascorbic acid (AsA) can stabilize levodopa and carbidopa solutions; however, the effect of AsA on the degradation of levodopa and carbidopa induced by MgO has not been fully investigated."
Ref: onlinelibrary.wiley.com/doi...
There seems to me to be a possibility of the following happening:
Magnesium oxide leads to less carbidopa, leads to less levodopa, leads to more constipation, leads to the patient taking more magnesium oxide, and so on.
Now let's be clear this is pure conjecture on my part, but it may go part of the way to explaining differences in the efficacy of a patient's drug regimen and differences between patients.
John