The gluteus maximus — commonly referred to as the buttocks — is the largest muscle in the human body. Intramuscular injections are the safest, easiest and best-tolerated of all injection routes, according to Brookside Associates. Consequently, injections in this muscle should not be painful.
Injections that are given in the muscle are preferred when the drug administered irritates subcutaneous tissue. The Center For Disease Control recommends that inactivated vaccines containing an adjuvant are better off injected into the muscle rather than under the skin. If these inactivated vaccines are injected into the skin, chances for irritation, inflammation and granuloma formation can increase.