There are two main classes of drug administration routes: enteral and parenteral. Enteral routes involve placing drugs directly into the gastrointestinal tract, including orally, sublingually, and rectally. Parenteral routes bypass the gastrointestinal tract, such as intravenous, intramuscular, subcutaneous, and inhalation. The route chosen affects how quickly the drug acts by determining the rate and extent of drug absorption into systemic circulation.