Scientists and doctors have not found any toxic or adverse effects due to consumption of large amounts of vitamin B12 from food or from supplements, according to the Linus Pauling Institute at Oregon State University. Patients consumed as much as 2 milligrams by mouth on a daily basis without significant side effects during a study in 1998. Some of the study's participants received a monthly injection of 1 milligram of vitamin B12 without any ill effects.
Scientists believe the human body absorbs just a small percentage of vitamin B12, even when someone consumes a large amount of the nutrient, explains the Linus Pauling Institute. Therefore, the U.S. Food and Nutrition Board has not set a tolerable upper intake level, as of 2015. Drugs may lower someone's absorption of vitamin B12, so doctors may prescribe or suggest a supplement to make sure a patient maintains healthy levels of the nutrient.
Adults should consume 2.4 micrograms of vitamin B12 per day, notes the Linus Pauling Institute. Foods that contain high amounts of vitamin B12 include steamed clams, mussels and Alaskan king crab. Beef, salmon and poached eggs also have large amounts of vitamin B12. Roasted turkey and chicken contain the micronutrient as well. People who want to take oral supplements of vitamin B12 should look for the words "cyanocobalamin" or "methylcobalamin" on the label.