In part, Hinz et al. base their case against carbidopa on the 328.7% increase in PD death rates between 1976 and 2011, 1975 being the date of introduction of carbidopa as a PD therapy (as part of Sinemet). Furthermore, they report that between 1958 and 1975 PD death rates declined and that this was attributed to the introduction of levodopa.

At a first glance, the data appears to present a strong case that the increase in mortality rates has been caused by carbidopa. But, in my opinion, additional data that the authors supply weakens the case considerably.

The authors include a graph (Figure 2, white males age 65 and over) that shows an increase of 311% in PD death rate between 1981 and 2010. So, most of the increase occurred years after the introduction of carbidopa. This suggests that the increase was not due to a delayed response to the migration of patients from plain levodopa to levodopa/carbidopa.

A feature of the graph is that, as far as I can "eye-ball" it, the death rate increased in 27 of the 30 years. It is difficult to see how the almost year on year increase can be attributed to the introduction of cabidopa 6 years before the start of the sequence.

Reference

[1] "The Parkinson’s disease death rate: carbidopa and vitamin B6"
Hinz M., Stein A., Cole T.
Clinical Pharmacology: Advances and Applications
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John