What Is Vitamin B Complex For? The Eight B Vitamins Explained

Vitamin B complex is a supplement that bundles the eight B vitamins, and it is for one thing: covering a diet that is short of them across the board, so that the body can keep turning food into energy, making red blood cells and running its nerves, skin and brain as it should. Each B vitamin has its own authorised roles in the UK, from thiamine's part in heart function to B12's in red blood cell formation, and all eight are water-soluble, so excess leaves in urine and little is stored except B12.¹ Most people eating a mixed diet get enough of all of them. The people who do not are vegans and over-65s for B12, women planning pregnancy for folate, heavy drinkers for thiamine, and anyone on a very restricted diet for the set. This is the guide to what each one does, how much you need, and whether a complex or a single vitamin is the right answer.

• The eight: B1 thiamine, B2 riboflavin, B3 niacin, B5 pantothenic acid, B6 pyridoxine, B7 biotin, B9 folate, B12 cobalamin
• Shared job: cofactors for energy-yielding metabolism; several also for the nervous system, red blood cells, skin and psychological function
• Who runs short in the UK: B12 (vegans, over-65s, metformin), folate (pregnancy, poor diet), thiamine (heavy drinking), riboflavin (teenage girls)
• Who a complex suits: a diet thin across several B vitamins at once; a single vitamin suits one specific gap
• Limits: B6 at or below about 10 mg a day long term; folic acid below 1 mg unless prescribed

The eight B vitamins and what each does

The wording below is the UK-authorised wording for each vitamin, with the adult reference nutrient intake, so you can see what a label may claim and what the evidence supports.^{1, 2}

• Thiamine (B1), 1.0 mg men, 0.8 mg women. Contributes to normal energy-yielding metabolism, normal functioning of the nervous system, normal psychological function and normal heart function. Short in heavy drinkers and people with very poor diets; severe deficiency is beriberi and Wernicke's encephalopathy.³ Wholegrains, pork, peas, beans, nuts, fortified cereals.
• Riboflavin (B2), 1.3 mg men, 1.1 mg women. Normal energy-yielding metabolism, maintenance of normal skin, vision, red blood cells and the nervous system, protection of cells from oxidative stress, reduction of tiredness and fatigue, and normal iron metabolism. Marginal in many teenage girls and young women who avoid dairy. Milk, yoghurt, eggs, fortified cereals, mushrooms. It turns urine bright yellow.
• Niacin (B3), 16.5 mg men, 13.2 mg women. Normal energy-yielding metabolism, normal functioning of the nervous system, normal psychological function, maintenance of normal skin and mucous membranes, reduction of tiredness and fatigue. Meat, fish, wheat flour, eggs. The nicotinic acid form flushes the skin above about 30 mg; nicotinamide does not.
• Pantothenic acid (B5), no UK reference intake, about 3 to 7 mg typical. Normal energy-yielding metabolism, normal mental performance, normal synthesis of steroid hormones, vitamin D and some neurotransmitters, reduction of tiredness and fatigue. In almost every food; deficiency is practically unknown.
• Vitamin B6, 1.4 mg men, 1.2 mg women. Normal energy-yielding metabolism, normal protein and glycogen metabolism, normal functioning of the nervous system and immune system, normal red blood cell formation, normal psychological function, regulation of hormonal activity, normal homocysteine metabolism, reduction of tiredness and fatigue. Pork, poultry, fish, bananas, potatoes, chickpeas, fortified cereals. The one B vitamin with a real upper limit: about 10 mg a day long term, because excess damages nerves.⁴
• Biotin (B7), no UK reference intake, 10 to 200 micrograms is safe. Normal energy-yielding metabolism, normal functioning of the nervous system, normal macronutrient metabolism, normal psychological function, maintenance of normal hair, skin and mucous membranes. Eggs, nuts, seeds, liver, wholegrains. Deficiency is rare; the hair claim applies to maintenance, not growth, and is biotin good for hair growth explains the gap.
• Folate (B9), 200 micrograms; 400 micrograms of folic acid before conception and until week 12 of pregnancy. Normal blood formation, normal amino acid synthesis, normal homocysteine metabolism, normal psychological function, normal immune function, reduction of tiredness and fatigue, a role in cell division, and maternal tissue growth during pregnancy. Green vegetables, pulses, oranges, fortified cereals. Folic acid in pregnancy has the UK rules.
• Vitamin B12, 1.5 micrograms. Normal energy-yielding metabolism, normal functioning of the nervous system, normal homocysteine metabolism, normal psychological function, normal red blood cell formation, normal immune function, reduction of tiredness and fatigue, a role in cell division. Only in animal foods and fortified products, which is why vegans, over-65s and people on metformin or acid suppressants run short.⁵ What vitamin B12 is good for goes further.

Claims that niacin "eases the pain of arthritis" or that B complex is "highly effective for weight loss" circulate online. Neither is authorised and neither is supported; B vitamins contain no calories and do not change weight in either direction.

What they have in common

Every B vitamin is a cofactor, a small molecule that an enzyme needs to work. Most of them sit in the chain of reactions that releases energy from carbohydrate, fat and protein, which is what "contributes to normal energy-yielding metabolism" means and why the whole set is sold as an "energy" supplement. In practice, if you are short of one, your cells extract energy less efficiently and you feel tired, and the groups listed above show how often a UK diet is marginal in at least one of them; a complex supports normal energy release rather than acting as a stimulant.⁶ Tiredness with bone or muscle pain is a vitamin D question instead: signs of vitamin D deficiency lists its signs and how to check your vitamin D levels the test. Several are also needed to make red blood cells (B12, folate, B6, riboflavin), to build and repair nerves (B12, B6, thiamine) and to make the chemicals that regulate mood (B6, folate, B12), which is where the authorised "normal psychological function" wording comes from.

Who should take a B complex?

People whose diet is thin across several of the eight: on a very low-calorie or restrictive plan, drinking heavily, recovering from illness or surgery, eating mostly processed food with few wholegrains, pulses, vegetables and dairy, or older and eating little. A complex at one to three times the reference intakes covers the shortfall without going near any limit. People with one specific gap can take the single vitamin, or a complex that states a useful dose of it: a vegan needs a dependable daily B12 source of 10 micrograms or more; a woman planning pregnancy needs 400 micrograms of folic acid; a heavy drinker may be prescribed thiamine. Which is better, B12 or B complex sets out that choice, and is it OK to take B complex every day covers dose and the two limits.

Is B complex safe, and when should you take it?

Safe at label doses for almost everyone, with vitamin B6 the one to watch: keep the total from all products at or below about 10 mg a day for long-term use, because higher doses over months can cause numbness and tingling in the hands and feet.⁴ Folic acid above 1 mg can mask a B12 deficiency. Take it at any time of day, with food for comfort; B vitamins are not stimulants and the "morning for energy" rule is folklore, as when should B complex be taken explains. People on levodopa, methotrexate or anti-epileptics should follow their prescriber's instructions.

A B-complex supplement supplies all eight in one capsule or tablet, with the folic acid and B12 figures stated on the label; the B vitamins in it contribute to normal energy-yielding metabolism, normal functioning of the nervous system and the reduction of tiredness and fatigue. For a single gap, a single-vitamin product such as vitamin B12 on its own is the more targeted choice.

What we don't know

• Whether B vitamins above the reference intake improve mood or cognition in people who are not deficient. Trials are mixed.⁶
• The lowest harmful long-term dose of B6. The 10 mg guidance is precautionary.⁴

Frequently asked questions

What is vitamin B complex used for?

Covering a diet that is short of several B vitamins at once, so the body can keep turning food into energy, making red blood cells and running its nerves and skin. It supports normal energy release rather than acting as a stimulant.

What are the eight B vitamins?

Thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7), folate (B9) and cobalamin (B12).

Who should take a B complex?

People on restrictive diets, heavy drinkers, people recovering from illness, and older adults eating little. Vegans, women planning pregnancy and people on metformin need a dependable dose of one specific vitamin, from a single product or a complex that states it on the label.

Does B complex make you gain weight?

No. B vitamins have no calories and no effect on weight in either direction; the claim that they help weight loss is equally unfounded.

Sources

1. NHS. B vitamins and folic acid. nhs.uk; Great Britain nutrition and health claims register.
2. Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals, 2003. cot.food.gov.uk.
3. Chandrakumar A, Bhardwaj A, 't Jong GW. Review of thiamine deficiency disorders: Wernicke encephalopathy and Korsakoff psychosis. Journal of Basic and Clinical Physiology and Pharmacology, 2018. PubMed 30281514.
4. Hadtstein F, Vrolijk M. Vitamin B6-induced neuropathy: exploring the mechanisms of pyridoxine toxicity. Advances in Nutrition, 2021. PubMed 33912895.
5. Green R, et al. Vitamin B12 deficiency. Nature Reviews Disease Primers, 2017. PubMed 28660890.
6. Kennedy DO, et al. Vitamins and psychological functioning: a mobile phone assessment of the effects of a B vitamin complex, vitamin C and minerals on cognitive performance and subjective mood and energy. Human Psychopharmacology, 2011. PubMed 21751253.

This article is general information, not medical advice. If you take levodopa, methotrexate or anti-epileptic medicines, or have an unexplained anaemia, ask your GP or pharmacist before taking B vitamins.