If a product is erroneously given subcutaneously (under the skin) instead of intramuscularly (into the muscle), the most common side effects include pain, swelling, and inflammation at the injection site. This can sometimes progress to cellulitis, which is an infection of the skin and subcutaneous tissues, or abscess formation, which is a localized collection of pus. In addition to these local reactions, there may also be systemic side effects if the medication is absorbed too quickly or in too high of a concentration. This can lead to symptoms such as fever, lethargy, vomiting, or diarrhea.