B complex vitamins have very few negative side effects when taken under a physician's recommendations. In fact, they are responsible for processing amino acids in the body, which promote cell growth and build the immune system. Nevertheless, if B complex vitamins are taken in too high a dose or if an allergy is present they can have serious, negative side effects.
Heart palpitation, insomnia, agitation, high blood pressure, skin eruptions and hypersensitivity can occur if B1 is taken in excess.
Overdoses of B12 can cause nausea, vomiting, fatigue, anemia and low blood pressure.
Vitamin B3 or Niacin
Taking too much B3 can cause nausea, vomiting, headaches, flushing of the skin, itchiness and a throbbing head. Try Niacinamide as an alternative.
Edema, fatigue, joint pain, dehydration and depression are negative side effects of too much B5 in the system.
High doses taken over a long period of time may cause numbness and tingling in fingers and toes as well as loss of muscle coordination. Depression and mood swings has also been known to occur.
B9 or Folic Acid
Side effects of taking too much Folic Acid can cause kidney damage, nausea, loss of appetite and abdominal bloating.
Too much B12 will have such negative side effects as numbness or tingling in the face as well as anxiety.