Because B12 has a low potential for toxicity, there is no upper level of intake for the vitamin, according to the National Institutes of Health Office of Dietary Supplements. The recommended daily intake is 2.4 micrograms for men and women and can be gained through diet and supplementation.
With no adverse side effects reported with doses much higher than the recommended value, B12 can be administered at higher rates than the recommended value. B12 is most commonly found in animal products such as meats, poultry, fish, eggs and milk, notes the National Institutes of Health. Some cereals are also fortified with vitamin B12 so vegetarians are able to get adequate B12 from food sources.