Such reactions are generally mild and resolve within a few days

Cyanocobalamin is a manufactured form of vitamin B 12 used to treat vitamin B 12 deficiency.[2] The deficiency may occur in pernicious anemia, following surgical removal of the stomach, with fish tapeworm, or due to bowel cancer.[6] It is less preferred than hydroxocobalamin for treating vitamin B 12 deficiency.[4] It is used by mouth, by injection into a muscle, or as a nasal spray.[4][5] Cyanocobalamin is generally well tolerated.[7] Minor side effects may include diarrhea and itchiness.[8] Serious side effects may include anaphylaxis, low blood potassium, and heart failure.[8] Use is not recommended in those who are allergic to cobalt or have Leber's disease.[6] Vitamin B 12 is an essential nutrient meaning that it cannot be made by the body but is required for life.[9][7] Cyanocobalamin was first manufactured in the 1940s.[10] It is available as a generic medication and over the counter.[4][7] In 2019, it was the 155th most commonly prescribed medication in the United States, with more than 4 million prescriptions

Not diet-related If your vitamin B12 deficiency is not caused by a lack of vitamin B12 in your diet, you'll usually need to have an injection of hydroxocobalamin every 2 to 3 months for the rest of your life
Yes, there are specialized B12 injections like Tiaminal B12
It also contributes in essential processes of body like blood cell formation and DNA synthesis, so make sure your dog does not lack it anymore