The dosing instructions will depend on a number of factors, including whether you have a deficiency, your age, your overall health, and any underlying conditions that should be considered
Special Cases: Athletes, Vegans, and Those with Absorption Issues Athletes Some athletes opt for injections every 2-3 weeks due to higher energy demands
Some conditions, like pernicious anemia, make it hard for your body to absorb B12
Here are answers to some of the most common ones

Increased absolute concentrations of intracellular NAD + links with activation of NAD + -dependent histone deacetylases (e.g., sirtuins) that also mediate epigenetic regulation of gene expression[40] and adipocyte physiology.[45] Similarly, increased intracellular concentrations of SAM, a universal substrate for SAM-dependent histone methyltransferases, could have profound influence on cellular epigenetic modifications, including transcriptional regulation and the expression of genes that regulate adipogenesis and/or thermogenesis promoting browning of adipocytes (e.g., PPAR, PRDM16, UCP1, Wnt).[4648] NNMT protein expression is relatively lower in the murine brown adipose tissue (BAT) compared to the WAT [37] and nnmt (a WAT-selective gene [49, 50]) gene expression has been reported to be lower in the BAT of HFD fed mice compared to normal chow-fed mice.[49] Furthermore, NNMT activity is significantly higher in the white fat compared to brown adipose tissue and the other organs (e.g., liver, lungs) in DIO mice.[16] Hence, treatment with an NNMT inhibitor in DIO mice may less likely impact BAT or other tissue NNMT activity, but could be speculated to modulate thermogenic/adipogenic genes in the WAT
