B6 (pyroxidine) influences the immune and nervous systems and is needed to convert the amino acid tryptophan to the neurotransmitter serotonin. It is also replete with antioxidants needed for all neurotransmitter activity. So essential is B6 for the brain and nervous system that if your diet is short even 1 mg. your nervous system could go haywire. Even minor depletion produces abnormal functioning of several enzymes responsible for metabolizing a variety of nerve chemicals, including serotonin, dopamine, and GABA, which regulate behavior. As toxins from these neurotransmitters don’t get broken down, they build up and irritate the nerves. Fruits are naturally high in B6.

Serotonin and Drug Side Effects
Mood swings and depression, side effects of medications such as hormone replacement therapy, oral contraceptives, and anti-tuberculous drugs might be caused by drug-induced suppression of vitamin B6 metabolism that in turn under-produces serotonin.

Information taken from Anxiety: Hidden Causes.