B vitamins are commonly added to energy drinks and other functional drinks, B1, B2, B3 (Niacin), B5 (Pantothenic Acid), B6 and B12 being most frequently used.
Thiamin (vitamin B1) has several important functions including working with other B-group vitamins to help break down and release energy from food and keeping nerves and muscle tissue healthy.
Riboflavin (vitamin B2) has several functions including keeping skin, eyes and the nervous system healthy and helping the body release energy from carbohydrate
Niacin (vitamin B3) has several important functions including helping produce energy from food when eaten and helping keep the nervous and digestive systems healthy
Pantothenic acid (vitamin B5) has several functions, such as helping release energy from food when eaten.
Vitamin B6 has several important functions including allowing the body to use and store energy from protein and carbohydrates in food and helping form haemoglobin, the substance that carries oxygen around the body
Vitamin B12 has several important functions and is involved in making red blood cells and keeping the nervous system healthy, releasing energy from food eaten and processing folic acid.
For more information see NHS Choices