Which route of administration allows a substance to take effect almost instantly?

The route of administration that allows a substance to take effect almost instantly is inhalation. When a substance is inhaled, it enters the lungs and is rapidly absorbed into the bloodstream due to the large surface area of the alveoli, the tiny air sacs in the lungs. This allows for nearly immediate entry of the substance into the circulatory system, resulting in swift onset of effects.

Inhalation is particularly effective for substances like gases or volatile compounds, as they can quickly diffuse into the bloodstream. The immediate effects are often felt within seconds as the substance takes advantage of the direct access to the systemic circulation, bypassing slower routes of absorption that oral or injected administration may involve.

In contrast, other methods such as oral administration require digestion and absorption through the gastrointestinal tract, which slows the onset of effects. Injected substances do provide a rapid effect, but typically not as instantaneous as inhalation, depending on whether they are injected intramuscularly or intravenously. Snorting a substance does allow for quick absorption through the nasal mucosa, but it does not achieve the rapid effect seen with inhalation.