Structural Biochemistry/Stages of how a drug passes through the body

Most, if not all, people have taken medicine before. They intake medications that help relieve pain, sickness, or soreness. Through much research scientists were able to formulate the four basic stages of how medicine goes through one's body. The four stages are absorption, distribution, metabolism, and excretion. The entire process is sometimes abbreviated ADME.

Absorption
The first step of how a drug passes through one's body is by absorption. There are few avenues on how medication may enter the body. The most common way is "orally" either with a tablet (pill) or liquid (via a spoon, cup or syringe). At home, many people use this technique. Another way is "intramuscular", which means to injecting a medication into an arm muscle. For example, when it's that time of the year when the flu is epidemic, everyone usually gets their flu shot through this method. Another way a medication enters the body is "subcutaneous", which is under the skin. Another route is "intravenous", this usually happens with people who undergo chemotherapy. The medication is received through a vein in the body. The last method for a medication to enter the body is "transdermal", which actually isn't inserted into the body. An example of this technique would be wearing a skin patch. The medication is absorbed through the skin and enters the body through our skin pores.

Out of all the steps of how a drug passes though one's body, absorption is the hardest barrier to overcome because everyone is created differently. Some drugs may not be able to enter one's body if the body rejects the drug. If a medicine is meant to spread throughout the whole body, it needs to first be absorbed. The person's body must accept the medication for it to take effect. There may be complications of drug administration because the drug may not go through the correct pathway in the body.

Distribution
After the medication is absorbed, the next step of how a drug passes through the body is distribution. A drug enters through one side of the body, but is meant to heal another part of the body, which might be in the middle of the body (or somewhere where it is hard to inject directly). The bloodstream is what helps a drug travel from the surface of where the drug enters to its destination. One obstacle that a drug must overcome would be side effects. As mentioned above, we are all created differently. One drug that works in one person may or may not work in another person’s body. People may react to a drug differently having one or more side effects. Some people have allergies to certain drugs. There are many factors that may prevent distribution of a drug inside the body. The “presence of protein and fat molecules in the blood” can attack a drug and inhibit it from its purpose. There are other complications of drugs, especially if they’re meant to go to the “central nervous system (CNS)”, where the brain and spinal cord are. The CNS has an obstacle called the “blood-barrier”, which guard the CNS from dangerous drugs. Pharmacologists have found ways to insert substances past the blood-barrier.

Metabolism
Once a drug reaches its destination, the drug must be “metabolized”. A drug is metabolized normally in the liver. The liver is chosen above any other organ because it’s a “site of continuous and frenzied, yet carefully controlled, activity.” All substances that enter the blood stream go directly to the liver and at the location the drug is altered to fit the appropriate function in the body. The “biotransformations” are the products of the alteration that happens in the liver. They are also known as enzymes. Enzymes are certain proteins that have a specific ability that when bound to a substrate it may perform its function.

Excretion
Once the enzymes perform their function, then the products of the break down are less chemically active than the original molecule. These products are called metabolites. The liver is called the “detoxifying” organ for the reason of which it breaks down the enzyme. Sometimes drug metabolites may have chemical activities of their own, which may or may not supersede the activity of the original drug. When the enzyme is done with its function, it may now go through excretion, which normally happens through the bowel.

Reference
Reference: Davis, Alison. (2006). Medicines By Design. National Institutes of Health, 11-13.