Some nutrients require another to function -- these are called cofactors, and appropriately, they should be taken together. Other nutrients might interact negatively with another nutrient or drugs, causing them to lose effectiveness or become harmful. Finally, there are nutrients that have no bearing on each other once they enter the body and can be taken together to efficiently meet your daily recommended value. Vitamin B-12 and niacin are two such vitamins, as they help the body in separate ways, but they can and often do appear together in foods and supplements.
Vitamin B-12, or cobalamin, enters the body naturally bound to protein from foods like meat, seafood and dairy products. Once the hydrochloric acid in the stomach unbinds it from protein, vitamin B-12 binds with an intrinsic factor, then absorbs into the bloodstream. The body uses it for nervous function, red blood cell maintenance and DNA production. With a diet that includes meat and dairy, it's likely you will consume the daily recommended dosage for adults of 2.4 mcg per day.
Niacin, or vitamin B-3, appears in seafood akin to vitamin B-12, but also in beets, beef kidney and liver, brewer's yeast, tuna and sunflower seeds. Fortified breads and cereals usually contain niacin as well, according to the University of Maryland Health Center. The recommended dosage is 14 mg for women and 16 mg for men. Deficiencies are rare, although they may occur under certain conditions, such as alcoholism.
Tryptophan is an essential amino acid, as the body does not produce it and therefore requires it from the foods you eat. Using iron, riboflavin and vitamin B-6, the body converts tryptophan into niacin. Tryptophan is also necessary to produce serotonin. Sources of tryptophan include dairy, meats, seafood, nuts, peanut butter, pumpkin seeds, soy and turkey. In essence, many of the sources of tryptophan are the same sources for vitamin B-12. Accordingly, although niacin and vitamin B-12 may not occur in the same foods directly, the body may still procure them both simultaneously from the foods you eat.
You can certainly take vitamin B-12 and niacin together. Not only do they appear in supplements together, they often appear together naturally in foods. Although the body requires them for different reasons that do not complement one another, it remains best to garner as many nutrients as possible from a single meal. However, taking niacin and vitamin B-12 together or separately will not produce any physiological benefits in the absence of a pre-existing deficiency in either vitamin.