Brief Answer: Vitamin B12 injections do not cause high heart rate. Detailed Answer: Hello, Thank you for using HCM. I understand your concern. Increased heart rate is not a known side effect of vitamin B 12 injections. The most probable cause of high heart rate, in your case, is anemia. As you are anemic, you have low or inadequate oxygen delivered to your tissues and organs, your hypoxic (lack of oxygen) tissues will send signals to your brain that it lacks oxygen and can t function properly. In response, your brain will stimulate your heart to pump faster (tachycardia) in order to compensate for the lack of oxygen. Once it will resolved totally and the blood full count will enter into normal ranges, your heart rate will go back into normal rate. If the heart rate continues to be high even after the anemia is resolved, than you should be tested for other causes of it such as: stress and anxiety, consumption of stimulants such as caffeine etc., hyperthyroidism, low blood pressure, dehydration, abnormal electrolyte levels etc. Meanwhile you should take plenty of water (2 - 3 liters per day). You should take an equilibrated diet. Try to avoid getting stressed or anxious. Hope this is helpful. Let me know if you need more clarifications. Otherwise please close and rate the answer. Kind Regards, Dr. Juarda