20 doses 0.25mL = 5mL bacteriostatic water, creating a 20mg/mL solution
These effects are normal and not usually a cause for concern
Yes, when administered by a trained professional, Biotin injections are safe
These amounts are considered safe because the human body, as a general matter, can safely store from 1,000 to 2,000 times the RDA, without negative consequence (anything more will generally be excreted in urine, per NCBI Bookshelf)
Conditions such as Crohns, coeliac disease or previous gut surgery, and the autoimmune cause behind pernicious anaemia, all affect how well you retain B12
Many people put the tiredness down to diabetes itself, or to a busy life, rather than to a vitamin their body can no longer absorb properly