Side effects of Vitamin B12:

usually nontoxic even in large doses.

Side Effects, Contraindications and Warnings

Cardiovascular : Caution should be used in patients undergoing angioplasty since an intravenous loading dose of folic acid, vitamin B6 and vitamin B12 followed by oral administration of folic acid 1.2mg plus vitamin B6 48mg and vitamin B12 60mcg taken daily after coronary stenting might actually increase restenosis rates. Due to the potential for harm this combination of vitamins should not be recommended for patients receiving coronary stents.

Dermatologic : Itching, rash, transitory exanthema, and urticaria have been reported. Vitamin B12 (20 micrograms/day) and pyridoxine (80mg/day) has been associated with cases of rosacea fulminans, characterized by intense erythema with nodules, papules, and pustules. Symptoms may persist for up to 4 months after the supplement is stopped, and may require treatment with systemic corticosteroids and topical therapy.

Gastrointestinal : Diarrhea has been reported.

Hematologic : Peripheral vascular thrombosis has been reported. Treatment of vitamin B12 deficiency can unmask polycythemia vera, which is characterized by an increase in blood volume and the number of red blood cells. The correction of megaloblastic anemia with vitamin B12 can result in fatal hypokalemia and gout in susceptible individuals, and it can obscure folate deficiency in megaloblastic anemia. Caution is warranted.

Lebers disease : Vitamin B12 is contraindicated in early Lebers disease, which is hereditary optic nerve atrophy. Vitamin B12 can cause severe and swift optic atrophy.

Pregnancy and Breastfeeding

Vitamin B12 is likely safe when used orally in amounts that do not exceed the recommended dietary allowance (RDA). The RDA for vitamin B12 in pregnant women is 2.6mcg per day and 2.8mcg during lactation periods.

維生素 B12 的副作用：

即使大劑量通常也是無毒的。

副作用、禁忌症和警告

心血管：接受血管成形術的患者應謹慎，因為在冠狀動脈支架植入術後靜脈注射負荷劑量的葉酸、維生素B6 和維生素B12，然後每天口服葉酸1.2 mg 加維生素B6 48 mg 和維生素B12 60 mcg實際上可能會增加再狹窄由於潛在危害，不建議接受冠狀動脈支架植入的患者使用這種維生素組合。

皮膚科：已有搔癢、皮疹、短暫性皮疹和蕁麻疹的報告。維生素B12（20 微克/天）和吡哆醇（80 毫克/天）與爆發性紅斑痤瘡病例相關，其特徵是嚴重紅斑，在結節、丘疹和膿皰停止補充後，症狀可能持續長達 4 個月，可能需要全身皮質類固醇和局部治療。

胃腸道：有腹瀉的報告。

血液學：有周邊血管血栓形成的報告。治療維生素B12 缺乏症可發現真性紅血球增多症，其特徵是血液容積和紅血球數量增加。以維生素B12 矯正巨幼細胞性貧血可導致致命的低血鉀症和易感個體患有痛風，並且可能掩蓋巨幼細胞性貧血的葉酸缺乏症。需要小心謹慎。

萊伯氏症：早期萊伯氏症是遺傳性視神經萎縮，維生素B12是禁忌的，維生素B12會導致嚴重且快速的視神經萎縮。

懷孕和哺乳
口服維生素 B12 的劑量不超過建議膳食攝取量 (RDA) 時可能是安全的。孕婦維生素 B12 的建議攝取量為每天 2.6 微克，哺乳期為每天 2.8 微克。