Which method of drug administration has the quickest effect?

Parenteral administration is the method that generally results in the quickest effect for drug delivery. This method involves bypassing the gastrointestinal tract, allowing the drug to enter the bloodstream directly through injections such as intravenous (IV), intramuscular (IM), or subcutaneous (SC) routes. This immediate access to the circulatory system facilitates rapid absorption and distribution of the drug, leading to quick therapeutic effects.

In contrast, oral administration requires the absorption of the drug through the digestive system, which takes longer due to various factors such as gastric emptying time, digestion, and first-pass metabolism in the liver. Inhalation can also be a rapid method of administration, particularly for certain medications like anesthetics or bronchodilators; however, it may not always provide the same immediacy as parenteral routes. Topical administration is typically the slowest, as it involves absorption through the skin, which can vary significantly based on the formulation and the area of application.

Therefore, due to the direct entry into the bloodstream and avoidance of metabolic processing before therapeutic action, parenteral administration is recognized for offering the fastest onset of drug effect.