The selection of a drug's delivery route depends upon its physicochemical properties, including lipid or water solubility and ionization, as well as the therapeutic requirement, such as immediate or sustained effect. These routes can be divided into three primary categories: enteral, parenteral, and topical.

Enteral delivery involves administering drugs directly through swallowing, sublingual placement, or buccal application. Orally administered drugs predominantly navigate the gastrointestinal tract for systemic absorption. However, this method has limitations, such as inadequate absorption, gastric irritation, instability in low gastric pH, and metabolism by gut flora. Notably, most enteral drugs are designed to address these drawbacks, ultimately leading to good absorption. For instance, drug formulations have been developed with protective coatings to limit drug degradation in the stomach.

Parenteral delivery, on the other hand, involves the direct injection of drugs into the bloodstream, muscles, dermis, or subcutaneous tissue, providing varying degrees of bioavailability. Amongst various parenteral routes, intravenous administration offers the highest bioavailability and the fastest onset of action.

Lastly, topical administration involves applying medication to the skin or mucous membranes. This method generally targets local effects, but some drugs applied topically can have systemic effects. For example, transdermal fentanyl patches deliver drugs systemically over time.

In conclusion, the choice of drug delivery route is critical to ensuring effective treatment. Each route has unique advantages and limitations that must be considered.