Absorption, distribution, metabolism and elimination are the major steps involved in what can be referred to as the life cycle of a drug. Once a drug enters the human body, it follows a particular pathway which eventually leads to the elimination of the drug.
Absorption is the first step and is linked to the process of drug liberation. Drug liberation is necessary for oral drugs and refers to the mechanism of a drug in pill form being released from its capsule or tablet. The drug must undergo disintegration and dissolution before absorption occurs. For liquids taken orally, it refers to the process of the drug dissolving with the fluids of the digestive system. Absorption can take other routes other than the mouth, for example, gases are absorbed by the lungs. Furthermore, the time it takes for absorption depends upon the concentration of the drug being administered.
Absorption, distribution and elimination are all similar in that they require membranes to be passed by the drug. Remember that membranes are lipid in nature. The processes by which membranes can be crossed are:
- Passive diffusion
- Filtration via pores
- Active transport
Distribution refers to the perfusion of a drug into the appropriate organ or tissue. This usually requires the binding of a drug to a receptor and the ability of the drug to cross a membrane. Once the drug has accumulated into the site of interest, metabolism is the next step. Metabolism is also referred to as biotransformation and takes place in the liver. This usually results in the transformation of the drug into a less active chemical which can be eliminated from the body through the kidney, the final step. Additionally, drug interactions, particular foods and diseases, such as liver disease, can interfere with the process of elimination, causing a drug to have a longer effect than intended.