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can b12 injections make you shaky Feeling worse after Injection: Answering concerns Best B12 Shots in Chicago
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The reduced amount of oxygen reaching the brain might be to blame for the thinking and reasoning problems

excursions permitted to 15 to 30C (59 to 86F) [see USP Controlled Room Temperature]
[Anemia caused by vitamin B12 deficiency in subjects aged over 75 years: new hypothesis

Since B12 plays a role in energy metabolism, some people report feeling more alert after taking their supplement, though this effect is anecdotal and not scientifically established