B-complex
A B-complex supplement combines the water-soluble B vitamins, including thiamine, riboflavin, niacin, B6, folate and B12. They work as coenzymes in the reactions that release energy from carbohydrate, fat and protein, and in the one-carbon pathways that build DNA and red blood cells. Such products are used to correct or prevent deficiency of these vitamins and are taken with an eye to energy metabolism.
It is taken with food in the morning; these vitamins support daytime energy metabolism, and taking them with a meal reduces stomach upset. They are water-soluble, so what is not needed is excreted; riboflavin is strongly coloured and turns urine bright yellow, which is expected and harmless rather than a sign of a problem. The values that show whether the B vitamins are doing their job are vitamin B12, folate and homocysteine, since homocysteine rises when B12, folate or B6 are lacking and falls as they are replaced.
Numbness, tingling, unsteady walking, a sore tongue or persistent unexplained fatigue can point to a B12 deficiency that a supplement may mask rather than resolve, so see a doctor for testing instead of continuing on guesswork.
Reference information, not medical advice. Discuss any change with your doctor.