First, the bad news. The Mayo Clinic looked at all the studies that had been published and found no evidence that vitamin B12 shots cause weight loss. Ironically, a side effect that people occasionally experience from B12 injections is swelling and weight gain.

Still, we’ve all heard of celebrities, co-workers and friends who swear by B12 shots as a weight-loss supplement. Why does it seem to work for some people? There are several reasons.

First, vitamin B12 shots speed up the metabolism. This in itself can promote weight loss, especially if your metabolism has been sluggish due to dietary deficiencies. 

Second, a few people who take vitamin B12 shots experience the side effects of nausea, vomiting, and diarrhea. Since most people don’t eat much when they’re feeling sick to their stomachs, they may lose weight. Needless to say, that’s not the healthiest way to do it.

Vitamin B12 deficiencies can cause vegetative depressive symptoms in some people. This means they eat more and move less. When the deficiency is corrected, their eating goes back to normal and they return to their daily routines. Some weight loss is a natural result.

It has also been hypothesized that B12 shots provide key nutrients so that dieters are less vulnerable to cravings. Another hypothesis is that the people who tend to become B12 deficient, like vegans and vegetarians, are already following a healthy diet. They would probably lose weight with or without the shots.

Finally, there’s the “placebo effect” to consider. The placebo effect kicks in when a patient believes a certain treatment will help him or her, even if that treatment has no medicinal value at all. At $65 or more a shot, B12 makes an expensive placebo, but if it works for you, go for it. The results are what matter.