It is hard to say with 100% accuracy how long a vitamin B12 injection lasts in the body. It all depends on how your body reacts to vitamin B12 shots and the frequency that you inject. In other words it’s a bit different for everyone.
Many studies show that people are unable to absorb or store sufficient amounts of vitamin B12. These absorption and storage issues stem from a variety of reasons and range from serious (medical) to benign (diet). Many people use B12 injections as opposed to oral supplements, as injections are more easily and quickly absorbed by the body.
However, one reputable source, written by Larry E. Johnson, MD, PHD at the University of Arkansas for Medical Sciences, explains that when people can properly absorb and store vitamin B12, it is stored within the body from between 3 to 5 years.
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