B-complex vitamins may cause temporarily flushing or an upset stomach, and the vitamin is used to supplement a poor diet and prevent or treat vitamin deficiency, states WebMD. Vitamin deficiency can be caused by pregnancy, certain illnesses and alcoholism.
Typical ingredients in a B-complex vitamin include thiamine, riboflavin, niacin/niacinamide, vitamin B-6, vitamin B-12, folic acid and pantothenic acid. Some B-complex vitamins also contain biotin, zinc, vitamin C or vitamin E. These vitamins are necessary to keep a person healthy, states WebMD.
Side effects rarely occur when taking vitamin B complex, but an allergic reaction is possible. Signs of an allergic reaction include itching or swelling of the face, tongue or throat; trouble breathing; severe dizziness; and rash, according to WebMD. If an allergic reaction is suspected, a physician should be contacted immediately.
People with liver problems, diabetes or a vitamin B-12 deficiency should consult a health care provider before taking vitamin B complex. Liquid vitamin B complex may contain sugar or alcohol, so those who are sensitive to either should avoid taking it. B-complex vitamins do pass through breast milk, but it considered to be safe, as of 2015. However, nursing women may want to speak to a doctor before taking B-complex vitamins, notes WebMD.