If you add 2mL, the concentration is 5,000mcg/mL
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Are Vitamin D injections better than oral supplements
Yes - the reasons for stinging are usually either swabbed area not dry, a drop of B12 at the tip of the needle (flick syringe and wait a moment) or B12 ampoule not warmed up just before injecting (especially if using fridge)
If you value precision control, cost savings, and flexibility, lyophilized powder with proper reconstitution is the better choice