The first stage of ADME is A, absorption. Medicines are absorbed when they travel from the site of administration into the body's circulation. A few of the most common ways to take medication are oral (like swallowing a pill), intramuscular (getting a flu shot in the arm), subcutaneous (injecting insulin just under the skin), intravenous (receiving chemotherapy through a vein) or transdermal (wearing a skin patch). Medicines taken by mouth are pushed through a special blood vessel leading from the digestive track to the liver, where a large amount of medicine is broken down. Other routes of bypass the liver, entering the bloodstream directly or via skin or lungs. Stage two of ADME is D, distribution. Most often, the blood stream is what carries the medicines through out the body. During this step is when the side effects can occur. For example a pain reliever might the target might be a sore muscle in the leg; irritation of the stomach could be the side effect.
After a medicine has been distributed it begins to break done, or metabolized, the M in ADME. Everything that enters the blood stream whether swallowed, injected, inhaled or absorbed through the skin it is carried to the liver. There, the substances are chemically pummeled, twisted, cut apart, stuck together and transformed by proteins called enzymes.
The drug undergoes the final stage excretion , the E in ADME. This removal happens via urine or feces. By measuring the amount of medicine in urine or blood, clinical pharmacologists can calculate how a person is processing the medicine. If the medicine is being eliminated relatively quickly then a higher dose might be required.
All in all I find this subject very interesting, I find it amazing how the body works in so many different ways.