Of those, 93,615 (64 per cent) either had tests confirm they already had normal B12 levels, or they weren't tested at all
Several laboratory tests should be performed prior to treatment with cyanocobalamin, including serum vitamin B12, folate, iron, hematocrit, and reticulocyte count
The length of time vitamin B12 to become effective depends on different health-related aspects that could slow down the process

Vitamin B12 shot may be prescribed to prevent or treat the following: Pernicious anemia (a condition in which there are low red blood cell counts due to an abnormality in B12 absorption by the intestines) Megaloblastic anemia (a type of anemia in which there are very large red blood cells called megaloblasts) Cognitive disorders, such as dementia (a disorder characterized by problems with memory, problem-solving abilities, language, etc.) Disorders that affect B12 absorption, such as pancreatic diseases, intestinal infections, Crohns disease, and tropical sprue Cardiovascular disease risk (by lowering blood homocysteine levels, which is a compound associated with an increased risk of heart problems) Multiple sclerosis symptoms Bells palsy recovery B12 deficiencies related to: Weight loss Vegan or restrictive diets Surgery to remove the part of the stomach or the ileum (the last part of the small intestine) Energy and endurance What are signs of vitamin B12 deficiency

The soreness correlates with injection technique: reviews mentioning "slow injection over 30 seconds" report 28% less soreness than those describing "quick push" technique (pattern analysis from 340 reviews specifying technique)
Therefore, its crucial to consult your doctor before taking the shots, in fact, before starting any medication