The forms of vitamin B12 used in the human body are methylcobalamin and 5-deoxyadenosyl cobalamin. These are also the two forms of cobalamin that cyanocobalamin, common in most vitamin B12 supplements, is converted to.

There has never been any toxic or adverse vitamin B12 side effects associated with large intakes of the vitamin. Whether from food or supplements vitamin B12 side effects are nonexistent in healthy people. In fact, doses as high as 1mg (1000 mcg) daily by mouth or 1 mg monthly by intramuscular (IM) injection have been used to treat patients with pernicious anemia and not a single one of them ever complained of significant vitamin B12 side effects.

The enzyme, methionine synthase is dependent on folate and for this reason, its enzymatic function require methylcobalamin. Methionine synthase, for its part, is also required in the synthesis of several important substances found in the human body, specifically amino acids and methionine. Both these substances work in a number of sites within the DNA and RNA (2) of the cell. Because of this, scientists believe that vitamin B12 may play an important role in cancer prevention and decreasing the risk of cardiovascular diseases.

The reason behind the low level of toxicity or vitamin B12 side effect may have to do with the fact that the body can only absorb small amounts of vitamin B12, even if taken in large doses. Because of this low risk of vitamin B12 side effects, no tolerable upper intake level (UL) was set by the Food and Nutrition Board in 1998 when the RDA was revised.

Another such drug that has vitamin B12 side effect is nitrous oxide. The drug is commonly used anesthetic and it inhibits both vitamin B12 dependent enzymes. It can also produce many of the clinical features of vitamin B12 deficiency, such as megaloblastic anemia or neuropathy. But other than that, there are no known vitamin B12 side effects that are adverse enough to cause undue alarm.