How Long Do B12 Injections Last: 5 Ways to Extend Effects If I took vitamin B12 injections and folic acid along with magnesium, a B complex, and carnitine, but there was no improvement and my condition actually worsened with increased anxiety, does this mean You might need a B12 injection if you're constantly feeling tired, run down, or low on energy. B12 supports energy levels, metabolism, mood, and neurological health. to help your body feel its What Causes B12 Side Effects? B12 Shots at Home: How, Where & How Often to Inject Yourself AZ IV Medics