It all comes down to an enzyme called NNMT
Vitamin B7 (biotin) deficiency is associated with hair loss and brittle nails, but more research is needed to understand the possible benefits of B vitamins on hair, skin, and nails

Clinical presenting features of the condition include those relating to anaemia (e.g., pallor, weakness, and fatigue), gastro-intestinal symptoms (e.g., epigastric pain and glossitis), neurological symptoms (e.g., from numbness of extremities and impaired proprioception in subacute combined degeneration of the spinal cord to severe developmental delay, epilepsy, and coma in inherited intracellular cobalamin defects), psychiatric disturbances and neural tube defects in the newborn of a deficient mother ( In cases which arouse clinical suspicion, there is no gold standard single test for the diagnosis of this condition ( p mol/L cut-off in the UK ( Current guidelines for B12 replacement Vitamin B12 is available in four different formulationscyanocobalamin (CN-Cbl), hydroxocobalamin (OH-Cbl), methylcobalamin (methyl-Cbl), and adenosylcobalamin (adenosyl-Cbl)depending on the R residual group attached to the core cobalamin molecule

Replacement therapy for vitamin B12 deficiency: Comparison between the sublingual and oral route
After reconstitution store 28C and use within 20 days