Vitamin B-6 is a water-soluble compound that was discovered in 1930s during nutrition studies on rats. The vitamin was named pyridoxine to indicate its structural homology to pyridine. Later it was shown that vitamin B-6 could exist in two other, slightly different, chemical forms, termed pyridoxal and pyridoxamine. All three forms of vitamin B-6 are precursors of an activated compound known as pyridoxal 5'-phosphate (PLP), which plays a vital role as the cofactor of a large number of essential enzymes in the human body.

 Several B vitamins, vitamin B-6, are essential to the maintenance of normal nervous system function in adults.₍₂₎

P5P 

 Ideally, the body takes B-6 and converts it to P5P in the liver. It's the P5P the body really needs.

P5P performs a variety of functions in the body including but not limited to:

1)Breaking down and using fats, proteins, and carbohydrates
2)Making red blood cells and antibodies
3)Help the digestive and nervous systems function
4)Maintain healthy skin
5)Etc.

 Some bodies, however, have trouble breaking down the B-6 and converting it to P5P. Those susceptible to this deficiency include people with liver problems, some breast-fed infants, breastfeeding mothers, elderly persons, persons on a poor diet, and women on estrogen-containing oral contraceptive pills or other hormone therapy. This problem can be easily avoided by simply taking P5P. 

_{1)Wikipedia.com 7-16-06
2)Rosenberg IH, Miller JW. Nutritional factors in physical and cognitive functions of elderly people. Am J Clin Nutr 1992;55(suppl): 1237S–43S.[Abstract]}