The good news is that you may notice the first ones after just 2-3 weeks of use, which is a relatively quick time
Reference: Neelakantan H, Vance V, Wetzel MD, Wang HY, McHardy SF, Finnerty CC, Hommel JD, Watowich SJ (2018)
For patients on established B12 injection therapy, monitoring is based primarily on clinical response and haematological parameters rather than routine serum B12 measurements, which may remain very high without indicating harm or overtreatment
Buteau J, Foisy S, Joly E, et al
Loading and maintenance: Higher initial doses tapering to maintenance levels
Today it is available only by prescription, compounded by state-licensed 503A pharmacies or FDA-registered 503B outsourcing facilities, neither of which is independently reviewed or approved by the FDA as a finished drug product, most commonly prescribed off-label for adult GH decline or wellness/anti-aging use