Methylcobalamin is the coenzyme form of vitamin B12. Several studies have found that people can have neurological signs and symptoms of a specific Methylcobalamin deficiency, even when serum B12, and blood tests which measure adenosylcobalamin acitivity, are perfectly normal. This is in large part because, while the body can store adenosylcobalamin (the other coenzyme form of vitamin B12) in the liver and mitochondria, Methylcobalamin is located in fluids like the cerebrospinal fluid and plasma, and is urinated out rather than stored.

Some signs and symptoms of a vitamin B12 deficiency can include high homocysteine, pernicious anemia (a type of autoimmune disorder that destroys certain stomach cells), mood changes, memory problems and neurological symptoms due to nerve damage.

It is now suggested that vitamin B12 deficiencies occur more often from malabsorption than from low intake. This means that B12 deficiencies may be more prevalent than was previously thought.