Final answer:
Parenteral administration, specifically intravenous injection, results in rapid drug absorption into the bloodstream and is used when immediate drug effect is necessary, typically in healthcare settings.
Step-by-step explanation:
The method of drug delivery that has a rapid onset because the drug is absorbed directly into the bloodstream is known as parenteral administration. This includes intravenous (IV) or intramuscular (IM) injections that bypass the gastrointestinal (GI) tract. The plasma levels achieved by intravenous administration are substantially higher than those achieved by oral or intramuscular administration, which is essential in treating severe infections promptly and effectively. These routes are commonly executed in healthcare settings due to their required precision and monitoring.