Will vitamin b12 injections cause weight gain. I don't want to gain weight. But I feel better taking them?

Whoa. If you have been prescribed B12 injections or other suitable replacement therapy based on sound evidence (i.e., labs and a real diagnosis), and you refuse to take them, you will pay for it with your health, your mind, and ultimately your life. They actually do not cause weight gain. SIck people feeling better after treatment may eat more. I'm sorry you were given faulty information. Best wishes.