Taking vit b12 shots every mnth but after 15 days i feel weak. legs tingle..how long before i become fine. shld i take magnesium supplements as well?

Could take months: If you had nerve, brain, or spinal cord damage due to B-12 deficiency, then it may take 3-6 months to replenish deficit, and reverse most of the problems. OK to supplement oral B complex. Magnesium is useful if you are also deficient in that category.

Vit B12: How low is your Vitamin B12? You might need to restore your B12 storage with everyday injection for a week then every week for a month or 2. Taking Magnesium is not going to restore your Vitamin B12 but it's not harmless to take it if you can tolerate the side effect.