Which form of vitamin B12 is best to take?
For most people, the chemical form of the supplement matters little, as long as you get enough of it. If you eat a fully plant-based diet, supplementation or the use of fortified products is indispensable.
Read the full answer
The body uses vitamin B12 in two active forms: methylcobalamin and adenosylcobalamin. These forms are needed for two core processes in your metabolism, including the production of methionine (a building block for proteins) and the breakdown of fatty acids. All B12 you take in, whether from meat, fish or a supplement, is converted inside your body into one of these two active forms.
Most supplement products contain cyanocobalamin, a synthetic form that is stable and inexpensive. Your body has to convert it into the active forms, which works well for most people. Supplements containing methylcobalamin or adenosylcobalamin skip that conversion step, but there is no strong evidence that these forms work measurably better in healthy people. Which form is best for you therefore depends less on the chemical variant than on how you get your B12 in the first place.
Through food, beef and sheep meat and dairy, and fish and shellfish, are the most reliable sources. Plants do not normally contain meaningful amounts of B12. If you eat a fully plant-based diet, supplementation or the use of fortified products is essential, because a deficiency is virtually certain on an unfortified vegan diet.
Alternatives such as nasal sprays, skin patches and special encapsulation techniques have been studied for people who have difficulty absorbing B12 through the gut. The evidence that these methods work well is limited and inconsistent. Injections remain the most reliable route for people with serious absorption problems (such as a certain stomach condition that reduces intrinsic factor), but that choice should be made together with your doctor.
Claims are based on reviews and meta-analyses (PMID 33881359, 29216732, 42436036, 39317227, 29931214, 41010482). No direct comparative RCTs between cyanocobalamin, methylcobalamin and adenosylcobalamin were provided as a source; the claims about metabolic function are strong, but the comparison of supplement forms in healthy people has not been directly studied in the supplied studies.