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At the proper levels, the B vitamins are necessary for the human body to function. Too little or too much of them stresses cells and tissues and may lead to serious, life-threatening disease.

Signs and symptoms of consuming too much or too little of the B vitamins include:

Overdoses of B vitamins from food have not been documented. Instead, the limited reports of toxicity are the result of taking excessive supplements. In general, overdoses stress the kidneys, but rarely cause permanent damage. B6 is unique among the B vitamins because overdoses are known to cause nerve damage. However, this damage is reversible when supplementation is stopped. Overdoses of folate may mask symptoms of B12 deficiency, possibly leading to permanent nerve damage.

In contrast to the relatively limited reports on vitamin B toxicity, a long list of conditions is related to their deficiency. They include:

• Beriberi (severe B1 deficiency). Can affect the cardiovascular system (wet beriberi) or the nervous and muscular systems (dry beriberi). Without treatment, beriberi can lead to paralysis, heart failure and death. Wernicke-Korsakoff Syndrome is also a thiamine deficiency (a brain disorder caused by the lack of vitamin B1).  Thiamine deficiencies are common in chronic abusers of alcohol.
  
  
• Ariboflavinosis (severe deficiency of B2). Rarely occurs alone and is treated in conjunction with other vitamin deficiencies.
  
  
• Pellagra (B3 deficiency). More common in places where diets are heavily dependent on corn. Without treatment, it will eventually lead to death.
  
  
• B5 deficiency. Rare because B5 occurs in so many foods. However, it can impact every system of the body.
  
  
• B6 deficiency. Mild deficiency is common, but severe cases are rare. Alcoholics and older adults are more likely to have inadequate vitamin B6 intakes because they have often have limited variety in their diet. Alcohol also promotes the destruction and loss of vitamin B6 from the body.
  
  
• Folate deficiency. Can cause a variety of conditions. General deficiency of folate may cause megaloblastic anemia and is indistinguishable from the body’s reaction to too little B12. During early pregnancy, folate deficiency in the mother can cause low birth weight, neural tube defects such as spina bifida, and separation of the placenta from the uterine wall. In children, folate deficiency can lead to slower than average growth. Folate deficiency has also been observed in alcoholics. Alcohol interferes with the absorption of folate and increases the excretion of folate by the kidney. In addition, many people who abuse alcohol have poor quality diets that do not provide the recommended intake of folate.
  
  
• B12 deficiency. May take 20 years to develop because B12 is repeatedly recycled in the body. Long-term deficiency (pernicious anemia) may lead to paralysis as a result of the degeneration of nerve cells in the spine, brain and extremities. Strict vegetarians and vegans are at a greater risk of developing B12 deficiency than lacto-ovo vegetarians and non-vegetarians because natural food sources of vitamin B12 are limited to foods that come from animals. Also, breast-fed infants of women who follow strict vegetarian diets have limited reserves of vitamin B12 and can develop a vitamin B12 deficiency.
  
  
• Biotin deficiency. Rare due to the wide variety of foods containing biotin. Reported cases have been linked to intravenous (I.V.) feeding without supplementation and excessive ingestion of raw egg whites (possibly consumption of 12 or more raw eggs a day).
  
  
• Choline deficiency. Rare due to the body’s ability to make this vitamin and its presence in many foods.