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Side Effects of Methylcobalamin

By Patrick Cameron ; Updated August 14, 2017

Methylcobalamin, commonly referred to as vitamin B12, is found in foods such as fish, shellfish, dairy and meat or can be taken as a supplement. Methylcobalamin helps to maintain nerve cells in the body and create new DNA, the substance that dictates what our bodies look like, and how they function. Although B12 is a recommended for cellular health, in certain situations it can cause side effects.


In some cases, methylcobalamin has been known to cause an itchy rash. The rash causes pustules to form on the skin. It is common for the pustules to last up to 4 months after methylcobalamin supplements have been discontinued.

Peripheral Vascular Thrombosis

Too much methylcobalamin has been known to uncover polycythemia vera, a condition in which the bone marrow makes too many red blood cells. Correcting the amount of methylcobalamin in the body by taking a supplement could result in a condition called hypokalemia, which is a potassium shortage and can lead to death.


If you are using methylcobalamin and are contemplating angioplasty (getting stents), you should discontinue taking methylcobalamin supplements immediately. During the angioplasty process, intravenous fluids contain a combination of folic acid, B6 and B12, or methylcobalamin is administered. After surgery, continuing to take a methylcobalamin supplement can increase the restenosis rate. In other words, too much methylcobalamin in the system will narrow the blood vessels and could be dangerous to coronary health.

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