If you've had a BAD experience with B12 shots, it's most likely that you do not tolerate or convert cyanocobalamin well (see if you can verify what form of B12 was in the shot that made you react poorly, and then stay away from that specific form if you can)
I have found Hevert to be the most stinging and a bit painful
A 2019 systematic review and meta-analysis published in the Journal of Clinical Medicine has found that intermittent fasting may reduce insulin resistance, improve glucose metabolism, and reduce body mass index ( 34 )
However, due to its synthetic nature it is also the least bioavailable and active (Hunt, Harrington and Robinson, 2014) Methylcobalamin: The first of our non-synthetic forms of B12 is methylcobalamin, a naturally occurring co-enzyme
The health of your gut microbiome is intrinsically linked to your metabolic health