A patient of Parkinsonism, Mr. Ghai noticed that the therapeutic effect of levodopa decreased when he was given another drug by his physician but no interaction was seen when he switched over to levodopa-carbidopa combination. The possible drug prescribed by his physician can be:

Pyridoxine is a component of vitamin B complex. Pyridoxine is a cofactor for the enzyme, dopa decarboxylase and therefore, administration of vitamin B complex can stimulate the activity of this enzyme. Dopa decarboxylase conves levo-dopa to dopamine. Increased formation of dopamine in the periphery is undesirable because it cannot cross blood brain barrier. Therefore, stimulation of dopa decarboxylase decreases the therapeutic effect of l-dopa. If the enzyme, dopa decarboxylase is already inhibited with carbidopa, there will be no interaction with pyridoxine.