Which parenteral administration has the slowest onset of action?

The intradermal route of administration has the slowest onset of action primarily due to the location and the nature of the drug absorption process. Intradermal injections deliver medication into the dermis, the layer of skin beneath the epidermis, where blood supply is less than in other areas, such as muscles or veins. The absorption of the drug from this layer into the systemic circulation is slower, as it has to pass through the dermal layers and then into the vascular supply.

In contrast, intravenous administration provides immediate access to the bloodstream, leading to a very rapid onset of action. Intramuscular injections can result in a faster onset compared to intradermal, as muscles have a higher vascular supply. Subcutaneous injections can also have a relatively quick absorption, but they are generally faster than intradermal injections due to better vascularization compared to the dermis.

Thus, the pharmacokinetics associated with intradermal administration leads to a slower and more gradual absorption of the medication, making it the route with the slowest onset of action among the options provided.