The best site to inject growth hormone treatment is into the stomach.
There are several reasons for this. First, injecting GH straight into a muscle would cause some of it to be catabolized by the muscle's high blood circulation and high temperature, which could account for up to 10% of total GH injected contained in muscles are lost before reaching any cells (called "muscle pull"). Injecting further down under the skin avoids those two barriers. Second, it relaxes tense muscles on either side of an injection site so that the needle can go in more easily. Third, since many people have extra fat around their stomachs and not a lot of loose flesh elsewhere on their bodies there's less risk that excess GH will escape.
"A comparison of subcutaneous and intramuscular administration of human growth hormone (hGH) and increased growth rate by daily injection of hGH in GH deficient children - PubMed" https://pubmed.ncbi.nlm.nih.gov/3197659/