Mine burns as well. A few things have helped: my body seems to be adjusting so the sting is milder now, I let the vial reach room temperature, I mix 1000 mg with 5 ml, thigh injections hurt less than abdomen ones, and I switched to a 31 g 4 mm needle instead of the 32 g I use for other peptides.
I have two problems with my NAD+ and wonder if others are seeing them too. First, it burns badly when I inject. Second, the vials start forming crystals within five days after I add the BAC water.