Name common routes of drug administration considered in PK.

• A. Intravenous (IV), oral (PO), subcutaneous (SC), and intramuscular (IM)
• B. Intravenous (IV), oral (PO), transdermal, inhalation
• C. Oral (PO), topical, intravenous (IV), sublingual
• D. Intrathecal, intramuscular (IM), inhalation, transdermal

Answer: (A)

In pharmacokinetics, you focus on how a drug enters the bloodstream and then how it is absorbed, distributed, metabolized, and eliminated. The four most commonly considered routes are intravenous, oral, subcutaneous, and intramuscular.

Intravenous administration delivers the drug directly into systemic circulation, so there is no absorption phase and bioavailability is effectively 100%; this makes it the reference point for clearance and distribution studies. Oral administration involves absorption from the gastrointestinal tract and often first-pass metabolism in the liver, creating a defined absorption phase and a potentially reduced, variable bioavailability that you must account for in PK modeling. Subcutaneous and intramuscular injections bypass the GI tract but differ in how quickly the drug is absorbed due to tissue vascularity and formulation; these parenteral routes are routinely studied to understand absorption rates and bioavailability under different conditions.

Other routes like transdermal, inhalation, sublingual, topical, or intrathecal are important in PK for specific drugs and contexts, but they introduce distinct and sometimes more complex absorption and distribution patterns that aren’t as universally characterized as the four above.