how long does the vitamin b12 injection last Are Shots Right for You? The Truth About B12 Injections:

We need better testingnot just serum B12, but also methylmalonic acid (MMA), homocysteine, and holotranscobalamin levels, which offer a more complete picture of B12 function

However, we do have some general recommendations for you to follow Our Recommendations Avoid Cyanocobalamin Cyano B12 is a synthetic, slightly-toxic, inactive form of B12 that is made with a cyanide donor

The NHS notes that taking up to 2,000 micrograms daily is unlikely to cause harm in most people, though there is generally no benefit to such excessive amounts without confirmed deficiency

Even a small but consistent loss of blood every month, when combined with decreased iron absorption from thyroid disease, is enough to slowly push someone into iron deficiency territory