Would vitamin B12 injections help with severe fatigue & brain fog? I've had every possibly test which all was normal other than low blood pressure.

Probably not but... It is crucial to find and treat the cause. I am sure you have not had every possible test. If you are low in B12 it can help but otherwise may not help much. You may not have adequate thyroid treatment- see http://tinyurl.com/6gvsxqu Low BP is often a sign of adrenal fatigue- see http://www.adrenalfatigue.org/ You might have heavy metal toxicity or chronic infections like Lyme disease.See comment: