can you overdose on vit b12 injections Shots at Home: How, Where & How Often to Inject Yourself Symptoms of B12 Overdose: Causes
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Without enough, you might feel tired, weak, and have nerve problems

We know that B12 recovery from bile would be a problem but this only contributes 5mcg to the picture, so I think its an open question

Increased levels of cobalamin after therapy confirm the effectiveness of vitamin b12 injection for dogs

For vegans, fortified plant-based milks, nutritional yeast and fortified cereals can provide B12