There are no known side effects associated with high intake levels of vitamin B12, states the Linus Pauling Institute at Oregon State University. Doses 1,000 times higher than the recommended dietary amount have been used with no significant reactions. There are no tolerable upper intake levels in place for B12.
Most Americans get enough vitamin B12 from a diet that includes fish, meat, eggs and dairy products, explains the National Institutes of Health Office of Dietary Supplements. As B12 is a water-soluble vitamin, the body absorbs the amount it needs and eliminates the rest, resulting in very low levels of toxicity. Supplements are usually used to treat B12 deficiencies of varying causes in doses as high as 2,000 micrograms.
Although there is no apparent danger associated with vitamin B12 toxicity, individuals should only take supplements and products containing B12 as directed, unless they are otherwise instructed by their physician. Certain medications can interact with vitamin B12 or decrease the body's absorption rate, including proton pump inhibitors for acid reflux and H2 receptor antagonists used to treat peptic ulcer disease, according to the Office of Dietary Supplements. Individuals are always urged to consult their doctor before adding any medication, vitamin or supplement to their daily routine.