B vitamins play different roles but their functions are inter-related and complementary. They work as coenzyme in several biochemical reactions to activate enzyme reaction of corresponding protein by creating holoenzyme. As holoenzyme they participate in the majority of cellular processes.   Although vitamin B1 and B6 participate more actively in the methionine cycle and citric acid cycle, which is the mitochondrial energy production process, the entire vitamin B group is involved in its successful execution. B vitamins also help to maintain the health of nervous system by playing a crucial role in the central nervous system and the peripheral nervous system functions.   Vitamin B1 (thiamine) serves as a coenzyme with transketolase enzyme in the pentose phosphate pathway of glycolysis. This pathway generates pentose sugar for synthesis of amino acid and nucleic acid and converts glucose to ribulose-5-phosphate via retrieving the carbon from the pentose phosphate shunt. Vitamin B2 (Riboflavin) plays an important role in many biologically important redox reactions such as energy producing, biosynthetic, detoxifying and electron scavenging pathways as a precursor of coenzyme flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD).   Riboflavin is also essential for the metabolism of homocysteine, as a cofactor for methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase (MTRR).   In the citric acid cycle, Vitamin B3 (Niacin) is involved in the formation of acetyl-CoA while conversion of its co-enzyme takes places from NAD+ (nicotinamide adenine dinucleotide) to NADH.    In the anabolic methionine cycle, niacin, in the form of NAD, is a necessary co-factor for the enzymes dihydrofolate reductase in the folate/tetrahydrobiopterin cycle and S-adenosylhomocysteine hydrolase. Vitamin B5 (Pantothenic Acid) is a component of coenzyme A. At least 70 enzymes utilize coenzyme A for the metabolism of fat, protein, carbohydrates and proper functioning of citric acid cycle.13 It is also involved in synthesis of several neural transmitters.   Vitamin B6 (Pyridoxine) acts as a necessary cofactor in the folate cycle and combines with serine hydroxymethyl transferase to effect the conversion of tetrahydrofolate(THF) to 5,10 methylene THF. B6 is a rate limiting cofactor in the synthesis of several neurotransmitters such as dopamine, serotonin, γ-aminobutyric acid (GABA), noradrenaline and the hormone melatonin. Even a minor deficiency of B6 can cause a substantial down-regulation of these neurotransmitters. B6 also plays an important role in immune functions and is also a bio-marker of inflammation with its down-regulation being associated with severe inflammation.   Source: https://www.empowerpharmacy.com/drugs/b-complex-injection