Vitamin B12 is an essential nutrient that your body uses for a variety of purposes, including proper nerve function and the formation of DNA molecules and red blood cells. However, humans don't absorb B12 especially well. Sublingual and injectable forms of B12 are designed to increase your body's absorption of the vitamin and boost its usefulness.
B12 Basics
Vitamin B12 has a large, highly complex structure that contains ions of the metal cobalt. If you consume B12 in food sources such as meat, fish or dairy, your body converts it to usable forms called methylcobalamin and 5-deoxyadenosyl cobalamin. Most B12 supplements contain a form of the vitamin called cyanocobalamin, which your body converts into either of the two main usable varieties. If you take a 1-mcg dose of oral B12, your body absorbs only 56 percent of the vitamin, according to the National Institutes of Health's Office of Dietary Supplements. If you take more than 1 mcg, the rate of absorption is even lower.
Sublingual B12
Sublingual B12 products are nonprescription lozenges or tablets that you place under your tongue until they dissolve. Manufacturers of these products claim that taking B12 in this way allows you to avoid the absorption problems associated with oral B12 and get more effective active doses of the vitamin. However, current evidence does not support these claims, and sublingual B12 appears to have the same effectiveness as oral versions of the vitamin, reports the NIH Office of Dietary Supplements.
Injectable B12
Injectable forms of B12 are prescription products that contain either cyanocobalamin or another form of the vitamin called hydroxocobalamin. They are typically used to treat a condition called pernicious anemia, which causes B12 absorption problems, as well as other absorption disorders and severe cases of vitamin B12 deficiency. By injecting B12 directly into a muscle in your body, your doctor can bypass the absorption problems associated with other types of the vitamin and ensure that you get enough of the nutrient to benefit your condition.
Safety
B12 is not toxic to the systems of healthy individuals, even when taken in large dosages, the Linus Pauling Institute reports. In oral forms, this low toxicity is due in part to your body's limited ability to absorb the vitamin. However, even when taken as an intramuscular injection in the treatment of pernicious anemia and other disorders, the vitamin has no significant toxic effects. For these reasons, there is no established upper limit for vitamin B12 intake.
Considerations
The Office of Dietary Supplements lists a number of medications that may decrease or alter the absorption of dietary or supplemental forms of B12, including H2 receptor agonists, proton pump inhibitors, metformin and chloramphenicol. Consult your doctor to learn more about the interactions between these drugs and B12. Apart from the presence of disorders that cause absorption problems, you have an increased risk for B12 deficiency if you are older, have undergone gastrointestinal surgery or strictly adhere to a vegetarian diet.
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