Very rare. Vitamin b1, which is frequently called thiamine, is part of the b complex of water-soluble vitamins required for proper nutrition. Because vitamin b1 is water soluble, taking too much of it means the excess is flushed from the body in urine. As a result, overdosing on vitamin b1 is extremely rare. Read more: http://www. Livestrong. Com/article/367247-vitamin-b1-overdose-symptoms/#ixzz1wh5siojc.