Your body relies on a vitamin B complex for a wide range of bodily functions, including development and growth. Enzymes, which are proteins that determine various chemical reactions in your body, rely on the various kinds of vitamin B for proper functioning. The B vitamins are vital for turning the food you eat into energy. Since B vitamins are water-soluble, excess is washed out through your urine. Though they are rare, overdoses and allergic reactions are possible.
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Thiamine, or B-1, can cause an allergic reaction that leads to skin rashes and irritation. Alcoholics and people with liver disease may have trouble processing thiamine, according to Medline Plus, a National Institutes of Health website. Thiamine is given as a supplement to treat digestive disorders such as diarrhea and poor appetite and to boost the immune system. A thiamine deficiency can lead to a brain disorder called peripheral neuritis. Naturally, thiamine is in food products such as beans, meats, nuts, yeast and cereal grains. Raw fish may destroy thiamine in the body, and the chemicals in tea and coffee can interfere with its production. Betel nuts, or areca, may cause a thiamine deficiency, and horsetail supplements destroy thiamine as it enters the stomach.
An allergic reaction to B-3, also called niacin, can lead to anaphylactic shock and death, according to Aetna InteliHealth. Niacin supplements typically are used to treat digestive disorders, high cholesterol, Alzheimer's disease and atherosclerosis. You may undergo other side effects if you are sensitive to niacin. Flushing, nausea, diarrhea, vomiting and itching may occur. A peptic ulcer may become irritated when you take niacin supplements. Blurred vision, macular edema and liver damage also may develop. If you have diabetes, niacin can raise your blood sugar levels. Other side effects can include dizziness, difficulty breathing, anxiety, dental pain and headaches. One of the most common sources of niacin is fortified cereal. It also is found in bananas, potatoes, garbanzo beans and chicken breast.
Pyridoxine, also called B-6, is used to treat clogged arteries, anemia and high cholesterol. It's used by pregnant women to treat morning sickness and by menstruating women to reduce the symptoms of PMS. Other uses of B-6 supplements include treatment for kidney stones, lung cancer, macular degeneration and seizures by infants. Pyridoxine is safe for most people, reports Medline Plus, but allergic reactions can include stomach disorders ranging from vomiting and loss of appetite to pain and nausea. It can make you sleepy if you have an adverse reaction to the supplements or cause tingling in your body. Long-term use of B-6 supplements can lead to nerve and brain complications.
Riboflavin, or vitamin B-2, can cause diarrhea when taken in large doses. If you are sensitive to B-2, it may cause your urine to turn an orange-yellow color or cause you to have an increase in urine output. Riboflavin is used to treat migraine headaches and deficiencies associated with cervical cancer. Low riboflavin levels also can cause acne, muscle cramps, blood disorders and carpal tunnel syndrome. It's also used for a range of conditions, such as cataracts, low immune system function, memory loss and ulcers. Athletes often use riboflavin to increase performance levels. Vitamin B-6 occurs naturally in nuts, eggs, green vegetables, meat and milk.