How long does it take feel better after b12 shot still feeling weak headache sleepy is it possible in any case that injections now work no tsh normal?

Not a panacea: B12 shots can be great but in my experience, do not result in people feeling immediately better that often. There are other things that cause these symptoms including Lyme disease, blood disorders, food sensitivities, sugar issues, adrenal issues, nutritional deficiencies like vitamin D, magnesium and more. Your doc can do a more thorough evaluation.

Not unexpected: B12 shot works only when one has a vit B12 deficiency and even then it will take many days if not weeks to experience improvement. You need to get checked for other conditions which may be causing your headaches and drowsiness.