Should I take vitamin B12 if my serum B12 level is high?

Common contributors to Vitamin B12 deficiency include:

  • vegetarian/vegan diet
  • stomach acid medications
  • antibiotics
  • oral contraceptives
  • aspirin (ASA)
  • blood sugar medications such as metformin 

Normal serum vitamin B12 levels range between 200-900 pg/mL. Serum B12 levels may not accurately reflect tissue B12 levels. What this means is that you could have a tissue level deficiency that is masked by serum level adequacy.

B12 deficiency symptoms include strange sensations, numbness, or tingling in the hands, legs, or feet difficulty walking (staggering, balance problems), difficulty thinking and reasoning (cognitive difficulties), or memory loss, weakness and fatigue.

From the Linus Pauling Institute:
“The neurologic symptoms of vitamin B12 deficiency include numbness and tingling of the hands and, more commonly, the feet; difficulty walking; memory loss; disorientation; and dementia with or without mood changes. Although the progression of neurologic complications is generally gradual, such symptoms may not be reversed with treatment of vitamin B12 deficiency, especially if they have been present for a long time. Neurologic complications are not always associated with megaloblastic anemia and are the only clinical symptom of vitamin B12 deficiency in about 25% of cases. Although vitamin B12 deficiency is known to damage the myelin sheath covering cranial, spinal, and peripheral nerves, the biochemical processes leading to neurological damage in vitamin B12 deficiency are not yet fully understood.”

According to the Merck Manual: “Large amounts of vitamin B12 seem to be nontoxic”, although it cautions against self-medicating with large amounts.  The Standing Committee report on the Scientific Evaluation of Dietary Reference Intakes and its Panel on Folate, Other B Vitamins, and Choline and Subcommittee on Upper Reference Levels of Nutrients, Food and Nutrition Board, Institute of Medicine also states “There is not sufficient scientific evidence to set a Tolerable Upper Intake Level (UL) for vitamin B12 at this time.”

Summary:

  • B12 deficiency features many of the same symptoms as fibromyalgia.
  • Neurological symptoms may be the only indicators of deficiency in 25% of patients, even in the presence of “normal” serum levels.
  • There is no upper tolerable limit set for Vitamin B12 because a level at which it becomes toxic has never been found. 

My conclusion:
Vitamin B12 constitutes a low risk intervention. Because nerves take a long time to repair themselves, it may be necessary to persist with treatment for a long period as well, especially if the neurological symptoms have been present for many years.  If you have one or more of the risk factors for B12 deficiency and you have ongoing neurological symptoms, it may be worthwhile supplementing B12.

References
http://www.clinchem.org/content/50/6/1043.short
http://lpi.oregonstate.edu/infocenter/vitamins/vitaminB12/
http://www.health.harvard.edu/blog/vitamin-b12-deficiency-can-be-sneaky-harmful-201301105780
http://www.merckmanuals.com/professional/nutritional_disorders/vitamin_deficiency_dependency_and_toxicity/vitamin_b12.html
http://books.nap.edu/openbook.php?record_id=6015&page=306