I was recently diagnosed with Vitamin B12 deficiency, which I have apparently had for some time now. I was scheduled to go in to get a B12 Shot and I needed to know how long it normally take to see an effect from these kind of shots. Would it take a couple of days or would it be more immediate? The answer I found was that this shot usually takes effect very quickly, as soon as a couple of minutes after it is administered. If you get too much vitamin B12 in your system, you will feel warm to hot. Shots are better than the tablet form because they can “kick start” your system. Also, most tablets do not get absorbed into the system, so you may ask your doctor about following the shot up with an easily absorbed liquid B12. Do not trust your medical safety to anyone by your physician. Once the shot gets into your system and you have been taking a follow up supplement, you should notice that you feel more awake, have more energy, and do not get sick as you normally would during flu season. Vitamin B12 should be taken in the morning to prevent insomnia due to taking it too late in the day. Iron is frequently prescribed with vitaminB12 to bolster flagging immune systems, particularly in high risk groups.