Which of the following vitamin should not be supplemented in a patient of parkinsonism on levodopa therapy
Thiamine
Pyridoxine
Cobalamine
Pantothenic acid
Pyridoxine acts as cofactor for Dapa decarboxylase enzyme that converts levodopa to dopamine. 
As peripheral conversion can cause postural hypotension, nausea & vomitting and also makes the therapy ineffective. Pyridoxine should not be supplemented.