Which route of drug administration offers 100% instant availability of the drug in the bloodstream?

The intravenous route of drug administration is the one that offers 100% instant availability of the drug in the bloodstream. When a medication is delivered intravenously, it bypasses all barriers to absorption, such as the gastrointestinal tract or the skin's layers. This direct entry into the circulatory system ensures that the entire dose reaches the bloodstream immediately, allowing for rapid therapeutic effects.

Intravenous administration is particularly advantageous in situations that require immediate drug action, as it allows for precise control over the drug's concentration in the blood. This is crucial in medical emergencies, anesthesiology, and when administering medications with a short duration of action.

In contrast, other routes like intramuscular or subcutaneous entail a slower absorption process as the drug must diffuse through tissues before reaching systemic circulation. Topical or transdermal routes also take time for the drug to penetrate the skin layers and enter the bloodstream, making them unsuitable for achieving immediate pharmacological effects.