For those who can’t absorb vitamin B12 from food, or who have pernicious anemia, doses that range from 300 to 2,000 micrograms (µg) a day have been used. To lower homocysteine levels, a vitamin B12 dose of 500 µg in combination with 0.5 to 5 mg (milligrams) of folic acid and 16.5 mg pyridoxine has been used. Typically, the dose of vitamin B12 is between one and 25 µg a day. Evidence
suggests that older adults can benefit from 10 to 25 µg a day.
In terms of side effects, even large doses of vitamin B12 administered orally or through injections are pretty safe. That said, very high doses of vitamin B12 can worsen acne symptoms. Vitamin B12, together with folic acid and vitamin B6, is linked to increasing the “restenosis” rates after coronary stenting. (This means the return of a narrowed blood vessel.) Therefore, if you are planning to
have this procedure done, check with your doctor before taking any of these vitamins.
Treatment of vitamin B12 deficiency can unmask a condition known as “polycythemia vera.” This is characterized by an increase in blood volume and the number of red blood cells leading to blockage of blood flow or peripheral vascular thrombosis.
Drugs that reduce B12 absorption include: proton pumps inhibitors (e.g. omeprazole, lansoprazole) for treating
gastric reflux; and other gastric acid inhibitors known as H2-receptor antagonists (e.g. “Zantac”) for treating chronic ulcers. I’d advise that, if you take these drugs, consider a vitamin B12 supplement.
Other drugs that inhibit B12 absorption include: cholestyramine used in the treatment of high cholesterol; choramphenicol; neomycin (antibiotics); colchicines (to fight gout); metformin (to fight diabetes); aminosalicylic acid; and anticonvulsants (phenytoin, phenobarbital, primidone).
From the supplement world, folic acid in very large doses will hide a vitamin B12 deficiency. In such a case, folic acid can in fact treat anemia, but at the same time allow neurological damage to continue. It’s very important that a doctor test your B12 levels before any folic acid is supplemented. Also, potassium supplements (especially potassium chloride) can reduce the absorption of vitamin
B12 in some people.
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