The classic signs of vitamin deficiency only occur in states of extreme depletion and are unreliable indicators for early treatment or prophylaxis of alcoholic patients at risk. Post-mortem findings demonstrate that thiamine (vitamin B1) deficiency sufficient to cause irreversible brain damage is not diagnosed ante mortem in 80-90% of these patients. The causes of vitamin deficiency are reviewed with special attention to the inhibition of oral thiamine hydrochloride absorption in man caused by malnutrition present in alcoholic patients or by the direct effects of ethanol on intestinal transport. As the condition of the patient misusing alcohol progresses, damage to brain, liver, gastrointestinal tract, and pancreas continue (with other factors discussed) to further compromise the patient. Decreased intake, malabsorption, reduced storage, and impaired utilization further reduce the chances of unaided recovery. Failure of large oral doses of thiamine hydrochloride to provide an effective treatment for Wernicke's encephalopathy emphasizes the need for adequate and rapid replacement of depleted brain thiamine levels by repeated parenteral therapy in adequate doses.