From experience, much depends on the brand/quality of the needles.So the vet allowed me to exchange the 18 for the 20 and also showed me a new way to make the "pocket"; I told him it didn't slip in easily, as it's supposed to, but I had to jab it hard to get it in. He said it's possible I had been also picking up the layer under the skin, which is tougher. I was horrified! My goal is to ease his discomfort; not cause him more.