As of 2015, vitamin B12 has no known toxicity levels when consumed from food or from supplementation, according to the Linus Pauling Institute. As such, the U.S. Food and Nutrition Board has set no upper intake level for vitamin B12. Doctors use intravenous doses of up to 1 milligram or oral doses of up to 2 milligrams of vitamin B12 to treat pernicious anemia in patients with a vitamin B12 deficiency.
Scientists believe the body's inability to absorb large amounts of vitamin B12 may explain the substance's low toxicity, the Linus Pauling Institute explains. Treatments for pernicious anemia have shown little to no side effects in patients when treated with high doses of vitamin B12. Several drugs, including proton-pump inhibitors, histamine2 (H2)-receptor antagonists, metformin, cholestyramine and colchicine, may decrease the body's ability to absorb this nutrient even further. Many of the medicines that prevent vitamin B12 absorption through food treat gastric problems such as acid reflux disease. Doctors may prescribe supplements of vitamin B12 for patients who take these drugs.
Anyone 14 years of age and older should receive 2.4 micrograms of vitamin B12, also known as cobalamin, per day as the recommended daily allowance, advises the University of Maryland Medical Center. Pregnant women should consume 2.6 micrograms per day, while breastfeeding women should take 2.8 micrograms daily.