Answer: Transdermal is one of the methods that drugs can be absorbed into the bloodstream.
Transdermal application is one way by which a drug can affect the body. Normally, the body has two protective adaptations to prevent substances from the outside world from affecting the brain: the skin and blood-brain barrier. Both of these protective mechanisms have developed over the course of our evolution. The skin is the first line of defense, and transdermal absorption is one way a drug can enter the body.
The skin is very repellent against water- based, or hydrophilic, substances. The skin secretes oils that prevent hydrophobic substances from permeating through. When a drug is applied transdermally, it must pass through the skin in one of two ways - either transcellularly or intercellularly.
Transcellular diffusion is difficult since the drug must pass through the cell. The cell consists of a hydrophobic lipid bilayer, which repels water, and a hydrophilic cytosol, which makes hydrophobic substances behave differently. The drug must then again pass through another hydrophobic lipid bilayer.
Intercellular diffusion is the movement of drug molecules between the layers of the cells. Even though the cellular material is only a thin section of the skin, the actual distance that a molecule must travel to penetrate the skin is almost an order of magnitude greater.
Drugs are often delivered via the application of an adhesive patch to the skin. A preparation of the drug then diffuses through the skin over the course of many hours. As such, transdermal application is a slow route of administration, as the drug may not have an effect for hours (compare this with an intravenous injection or inhalation, which can have an effect within seconds.)
The first transdermal drug delivery system approved for sale in the US was a preparation of scopolamine for motion sickness in 1979. Other common preparations include birth control patches to regulate hormone release, and Nicoderm CQ, which is used as a smoking cessation tool to help nicotine addicts minimize their drug use.