Vitamin B12 Injections Specialist in Langhorne, PA Question of the Day:, How long does it take for a B12 injection to rev up the engines of energy? , For many people, the boost can be felt within 2448 hours. Others may notice gradual changes Vitamin B12 Injections for Weight Loss & Side Effects (2025) Sizing Up the Risks? SA Grain How Vitamin B12 Injections Affect Your Body: Good or Bad? How Quickly Does a B12 Shot Work? Benefits & What to Expect