Skin popping is a route of administration of street drugs where they are injected or deposited under the skin. It is usually a depot injection, either subcutaneous or intradermal, and not an intramuscular injection. After deposition, the drug then diffuses slowly from the depot into the capillary networks, where it enters circulation. Skin popping is distinct from intravenous injection in that the latter deposits the drug directly into the bloodstream via a vein. Higher-potency prescription opioids, such as morphine, fentanyl, or meperidine can be injected subcutaneously, as can cocaine. (taken from Wikipedia)