Hello and thank you for your interesting question. Vitamin B12 stays in your system for only 2 to 3 days after receiving a shot, however the effects of a vitamin B12 shot last for far longer, such as blood count improvement, etc. If you are still feeling tired, I would expect one of the other nutrients to be depleted too. Have your doctor check your iron levels, thyroid, blood pressure and blood sugar to begin with. Correct these possible imbalances next and also consider whey protein and L-carnotine for strength and stamina.
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