PHENOBARBITAL AND PYRIDOXINE
Daily doses of pyridoxine 200 mg can cause reductions of 40 to 50% in phenobarbital levels in some patients. Primidone is metabolised to phenobarbital and may therefore be affected similarly, although this does not appear to have been studied.
Concurrent use should be monitored if large doses of pyridoxine are used, being alert for the need to increase the phenobarbital dosage. It seems unlikely that small doses (as in multivitamin preparations) will interact to any great extent.