“Several studies have shown that megadoses of the vitamin can lead to outbreaks of acne and rosacea, a skin condition that causes redness and pus-filled bumps on the face. Yet, it should be noted that most of these studies focused on high-dose injections rather than oral supplements (5, 6, 7).
There is also some evidence suggesting that high doses of B12 may lead to negative health outcomes in those with diabetes or kidney disease. One study found that people with diabetic nephropathy (loss of kidney function due to diabetes) experienced a more rapid decline in kidney function when supplemented with high-dose B vitamins, including 1 mg per day of B12. What’s more, the participants receiving the high-dose B vitamins had a greater risk of heart attack, stroke and death, compared to those receiving a placebo (8).
Another study in pregnant women showed that extremely high B12 levels due to vitamin supplements increased the risk of autism spectrum disorder in their offspring (9).“