Generally, skin covers the subcutaneous tissues and these subcutaneous tissues cover the underlying muscles. A longer needle (often 1 1/2 inch long) is inserted at a 90 degree angle for most intramuscular injections.
Care should be taken to avoid contact with the sciatic nerve. The sciatic nerve is long, it originates in the sacrum (sacral plexus) of the spine, and it courses through the muscles of the gluteal region, thigh, leg and foot. If the injection needle pierces the sciatic nerve there will usually be an intense “electrical” pain shooting down the leg.
The “rear end” (gluteal area) is commonly chosen for intramuscular injections. The upper outer quadrant of the rear end is least likely to contain the sciatic nerve, so this area should be chosen for these IM injections.
Alternative sites for IM injections often include the vastus lateralis (the largest of the 4 muscles that comprise the quadriceps muscle) located along the lateral thigh or the deltoid muscle (thick triangular muscle that covers the shoulder joint). Injections into these alternate sites tend to cause more discomfort than the gluteal region but they also may allow for more rapid absorption of administered medication.