Who Needs B12 Injections, How Much and How Often? InVita Wellness Vitamin B12 shots: Uses, benefits, and side effects A little insight to the importance of the b12 injections. My client hadn't been feeling well & had some bloods done with her GP. The results identified a severe deficiency in b12. How Long Does It Take For B12 Injections To Work? How Often Can You Get a B12 Shot for Maximum Health? If you have a low vitamin B12 level, injections can go a long way toward improving your health and well being. But if your B12 level is normal, don't fall for claims that