Levodopa (without carbidopa) may be antagonized by pyridoxine, Phenytoin/phenobarbital/primidone (folate may reduce levels), Methotrexate/trimethoprim (folate antagonists), Chloramphenicol (may blunt hematologic response to B12), Metformin and PPIs/H2 blockers (may reduce B12 absorption with long-term use)