10.1080/088800199277489 3 ArcanP.BozkurtO.CavusogluD.GencpnarP.HaspolatS.DumanO.et al (2020)

A physician investigating unexplained hypercobalaminemia will typically order some combination of the following: Complete blood count (CBC) with differential to check for abnormal red blood cell, white blood cell, or platelet counts Liver function tests (ALT, AST, ALP, bilirubin, albumin) to assess liver health Kidney function tests (creatinine, BUN, estimated GFR) to evaluate renal clearance Peripheral blood smear if a myeloproliferative disorder is suspected Abdominal imaging (ultrasound or CT) if liver or abdominal pathology is a concern Cancer-specific markers or screenings if clinical suspicion warrants them The NIH Office of Dietary Supplements fact sheet notes that observational data from a Danish registry analysis of 757,185 people found the adjusted 1-year cancer risk was 1.74 to 4.72 times higher among those with B12 above 813 pg/mL compared to those in the normal range of 203 to 813 pg/mL

Since this vitamin is essential for DNA synthesis, nerve cell health, red blood cell formation, and neurological function, a vitamin B12 injection really is a no-brainer
Its not something most people need to worry about due to how rare it is, but if unusual rashes occur after starting B12, speak with your doctor, and a skin patch test to check for allergies to metals may help to identify an issue
Methylation is a fundamental biochemical process involved in detoxification, hormone metabolism, neurotransmitter production, DNA repair, and gene expression