Vitamin B_1 (thiamin) has been clinically used in prophylaxis of Wernicke's encephalopathy. It is recognized that the parenteral vitamin B_1 administration is almost safe. However, the administration of usual amounts of vitamin B_1 occasionally causes life-threatening abnormality such as anaphylaxis. In this review, I introduce the previously reported data about the unexpected abnormalities after the parenteral administration of vitamin B_1. Moreover, I also introduce an available report which suggested that a non-toxic vitamin B_1 derivative with a benzoyl group may be converted into a toxic compound, O-benzoylthiamin, in animal body.