The same rule applies to the 28-day open-vial rule
[6] [3] [10] A mild deficiency may not cause any discernible symptoms, but moderate deficiency can cause various symptoms, such as feeling tired, lightheadedness, headaches, cold intolerance, pale skin, sore tongue, upset stomach, loss of appetite, hair and skin color changes, numbness or tingling (pins and needles) of the fingers and toes, and tinnitus, and vision, cognitive or psychological problems
Instructions for self-administering a vitamin B12 subcutaneous injection: We recommend seeking professional help before self-administering an injection, however, if you do wish to self-administer, here are some guidelines to help
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