The Linus Pauling Institute indicates there is no upper tolerable limit established for vitamin B12 by the U.S. Food and Nutrition Board due to a lack of toxic effects associated with high levels of vitamin B12 intake, as of October 2014. Daily doses as high as 2 milligrams have been ingested by patients with pernicious anemia with no serious side effects.
The reason for a lack of vitamin B12 toxicity is that the human body absorbs a small percentage of the nutrient when given orally, according to the Linus Pauling Institute. The National Institutes of Health reveals a healthy person who ingests 500 micrograms of vitamin B12 in an oral supplement only absorbs 10 micrograms of the vitamin.
The daily recommended allowance for adults is 2.4 micrograms of vitamin B12. Pregnant women should ingest 2.6 micrograms of the nutrient daily, according to the National Institutes of Health. Foods with high amounts of vitamin B12 include clams, beef, trout, salmon, fortified breakfast cereals, milk, eggs, cheese and ham. Dietary supplements with vitamin B12 usually list the ingredient as cyanocobalamin or methylcobalamin.
The Mayo Clinic observes vitamin B12 supplements are taken for canker sores, claudication, anemia, issues with mental performance, depression, cyanide poisoning and hereditary sideroblastic anemia. A method that has been used for preventing anemia is supplementing a diet with vitamin B12, iron and folic acid for up to 12 weeks .