Vitamin B12 Toxicity

Although vitamin B12 toxicity is uncommon, it is possible to develop side effects while taking the vitamin. Potential side effects include diarrhea, feelings of swelling over the entire body, blood clots in the legs, and signs of an allergic reaction. Although the risk for toxicity is low (even with high doses), you should report any possible side effects to your healthcare provider.

Is Vitamin B12 Toxic?

Vitamin B12 is a relatively non-toxic substance. Even high doses are unlikely to cause problems for most people. In fact, a maximum daily dosage has not been established for the vitamin, due to its non-toxic nature.

Signs of Vitamin B12 Toxicity

There are no signs of vitamin B12 toxicity, per se. There are a few rarely reported side effects that might be attributable to the vitamin, but such side effects are not necessarily related to the dose. These possible side effects include:
  • Diarrhea
  • Blood clots in the legs
  • Feelings of swelling over the entire body
  • Signs of an allergic reaction, such as:
    • Hives or a rash
    • Itching
    • Swelling of the lips, mouth, or throat
    • Wheezing or other difficulty breathing.

What Causes Toxicity?

Vitamin B12 toxicity is due to an obvious cause -- taking too much. The Recommended Dietary Allowance for vitamin B12 for adults ranges from 2.4 to 2.8 mcg daily, depending on several factors (see Vitamin B12 Dosage). For nutrients that can cause toxicity, a "Tolerable Upper Intake Level" (UL) is given. This is the maximum that can be taken from all sources, including the diet, without causing significant toxicity. There are no UL values for vitamin B12, since the risk of toxicity is quite low.

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Last reviewed by: Kristi Monson, PharmD
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