Adverse effects seldom occur after oral administration of thiamine and cyanocobalamin, but hypersensitivity reactions have been reported after parenteral administration. Hypersensitivity reactions to thiamine ranged from very mild to very rarely, anaphylactic shock. Hypersensitivity reactions have occurred rarely following oral administration of cyanocobalamin.
Long-term administration of large doses of pyridoxine is associated with the development of severe peripheral neuropathies.