transdermal administration

Transdermal administration delivers a drug through the unbroken skin and into the bloodstream, most familiarly through an adhesive patch stuck on the skin. The patch acts like a slow, steady tap: it releases drug continuously so that a roughly constant blood level is maintained for hours or days from a single application.

The skin's outer layer (the stratum corneum) is a tough, waxy barrier designed to keep things out, so only certain drugs can cross it: they must be potent (effective in tiny amounts), fat-soluble enough to dissolve through the waxy layer, and small enough to permeate. The drug diffuses passively down the concentration gradient from the patch reservoir, across the skin, into dermal capillaries. Because it enters the systemic circulation directly, it bypasses first-pass metabolism in the liver.

The strengths are convenience and steadiness: a patch avoids the peaks and troughs of repeated dosing, suits patients who cannot swallow, and is easy to stop by simply removing it (though some drug already in the skin keeps absorbing for a while). It is well suited to chronic conditions needing constant low-level drug delivery.

Drawbacks include slow onset (it can take hours to reach effective levels), local skin irritation, variable absorption depending on skin thickness and temperature (heat increases absorption and can cause overdose), and the limit that only a small number of suitable molecules qualify.