What about giving vit.b12 to diabetic complaining of numbness in there hands does it work?

Depends: Vitamin B12 will only resolve numbness if that numbness is from a vitamin B12 deficiency. A diabetic could have a B12 deficiency causing numbness, but they are more likely to have numbness from diabetic neuropathy - numbness from nerve damage from blood sugar levels being too high for too long. Before treatment, a search for the cause is necessary.

Not really: Not much has been shown to reverse diabetic neuropathy changes. Best bet is to optimize blood sugar control to prevent worsening. Alpha lipoic acid, acetylcarnitine, and fish oil (omega 3s) do have some evidence for benefit in diabetic neuropathy.

Depends on the cause: Numbness can occur from various reasons. Some potential causes include vitamin deficiency, nerve damage, nerve compression, and alcohol over consumption, side effects of different medications, or as a result of different systemic medical conditions. Get evaluated by a professional to determine the exact cause of the neuropathy in order to obtain the best possible treatment.