A deficiency of thiamine can cause weakness, fatigue, psychosis, and nerve damage. Thiamine deficiency in the U.S. and is most often seen in alcoholics, because heavy drinking limits the ability of the body to absorb this vitamin from foods. Since few alcoholics consume higher than normal amounts of thiamine to make up for the difference, they become deficient and may develop the disease called beriberi.

In severe deficiency, brain damage can occur. One type is called Korsakoff syndrome (confusion and loss of short-term memory); the other is Wernicke's disease (eye disturbances, unsteady gait, and confusion). Either or both of these conditions can occur in the same person.

There is no known toxicity associated with thiamine.