Which method of drug introduction has the quickest onset of action?

Inhalation is the method of drug introduction that has the quickest onset of action. When a substance is inhaled, it enters the lungs and is rapidly absorbed into the bloodstream through the alveoli. This process allows the drug to bypass the digestive system and liver metabolism, which can slow down the delivery of the drug to the central nervous system.

In contrast, ingestion involves the gastrointestinal tract, which requires time for digestion and absorption, leading to a slower onset of effects. Absorption can refer to various methods where the drug passes through biological membranes, but it typically does not match the speed of inhalation. Injection directly introduces the drug into the bloodstream, which is quite fast; however, inhalation can be even quicker due to the vast surface area and extensive blood supply in the lungs.

Therefore, the method of inhalation achieves rapid systemic circulation, making it the fastest route for drug action compared to other methods listed.