Why if b12 injections are given for "weightloss" did it cause a rapid heart rate and eating 3x more than normal to be satisfied?

No wt loss w/B12: As a family physician, i can assure you that i don't give B12 injections to cause weight loss in humans as there is no such link between the two. In other words, taking B12 whether as a shot or pill will not lead to weight loss. If you're low in b12, you may need B12 pills or shots, but that won't cause weight loss, rapid heart rate or increase in appetite. Latter two are most likely coincidence.

You're right: B12 will not cause you to lose weight. Unfortunately, that is a common scam in some places, particularly here in south florida. In fact, there are efforts to curb that with laws in florida. There is no evidence that B12 actually leads to weight loss. The rapid heart rate may just be a reaction to the injection. If a steroid is included, then it will increase your appetite.