Absorption, distribution, metabolism and excretion are the four basic stages of a medicine’s life in the body.


The most common ways that drugs can enter the body are:
•    Orally
•    Inhalation
•    Transdermal (administered through the skin like a skin patch or cream)
•    Subcutaneous (an injection just under the skin)
•    Intramuscular (a shot in a muscle)
•    Intravenous (through a vein)

In most cases, the highest absorption of any drug is when it is administered intravenously–80% to 100% of the drug is metabolized and used by the body.  The least effective absorption is orally–10% to 30% is metabolized and used by the body.


Generally, drugs are transported by the bloodstream throughout the body.  Depending on our DNA, they can affect the organs and other parts of the body and sometimes they don’t produce their intended effect.  Some drugs that are intended to provide pain relief can irritate the stomach.  Other drugs may create an undesired effect by being stored in an organ and then creating a problem for that organ.

Drugs intended to affect the central nervous system must penetrate the blood-brain barrier–tightly woven capillaries that protect the brain from unwanted substances—such as viruses.


Everything that enters the bloodstream—whether orally, by injection, by inhalation, absorbed through the skin, or produced by the body itself—is carried to the liver, the body’s largest internal organ. Most of the drugs go through the P450 pathway in the liver, which is essentially a group of enzymes that process the drugs.


Depending on our DNA, a drug will be excreted mostly through the urine or feces.