Introduction to Psychopharmacology - Part 2 (How are Drugs Processed?)

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The Processing of Drugs

Drug action does not only depend on the chemical structure of a drug but also on factors like the rate in which it can be absorbed by the body; this factor is known as bioavailability (1). There are five factors that contribute to bioavailability that constitute the pharmacokinetic elements of drug action (2). Pharmacokinetics is the study of how drugs are absorbed, dispersed, processed, and eliminated from the body (3). The five factors are:

1. Routes of administration.

This is the way in which drugs enter the body. There are two major divisions in which a drug can be administered: Enteral, which refers to passing thorugh the intestine or gastrointestinal tract (4), and parentenal, which is all the methods in which the drug do not passes through the gastrointestinal tract (5). We will cover five methods of administration.


The first one is by means of injection. We will cover several types of injections, the first one being intravenous (IV) injection. This one goes right in to the bloodstream so the drug effects are instantaneous (6). The second one is the peritoneal injection, which is administered in the peritoneal cavity (duh!). This is a semipermeable wall in the abdomen (7). The next injection is intramuscular, which is delivered into the muscles and the last one is the subcutaneous injection, which is delivered under the skin (8). The cartoon to the right depicts a peritoneal injection.




The second route to administer a drug is by oral administration. Researchers don't usually use this method for two reasons. The first one is that if the experimenter is dealing with animals, they (the animals, not the researchers) might not want to consume the drug because of its flavor. The other reason is that the chemical can be destroyed by a stomach acid (9). Nevertheless, this method can be used with humans if it is administered with a sublingual method (This means placed under the tongue). In this way, the drug enters the bloodstream, thus, not being affected by the stomach acid.


The third method is called intrarectal and it is administered through the anus (10). I hope you can understand why I didn't put a real photo here. It is important to note that because of they way it is introduced into the body, many researchers opt out of his method. Nevertheless, it is commonly used with humans. Inhalation is the fourth method. Drugs like marijuana and nicotine are usually smoked (9). This method is efficient in the sense that a lesser dose is needed for the same effect when compared to the other methods such as oral administration (7). The last method is called topical. This is applied to surfaces of the body such as the skin (8), colon, vagina, urethra, conjunctiva of the eye, or the nasopharynx (2).

 Each route of administration has advantages and disadvantages. For example, taking drugs orally makes it safer for the person (it avoids the use of needles), but it has a slower rate of absorption. In addition, taking a drug intravenously makes it reach the brain faster, resulting in quicker drug effects, but it is harder to self-administer and it requires sterile needles. Moreover, using an intramuscular injection creates a slow and steady rate of absorption, but it irritates the person at the site of the injection and it also requires sterile needles (2). 

 2. Absorption and Distribution.

 This refers to the way in which the drugs enter the blood plasma and is transported throughout the body (2).

 3. Binding

 This refers to the way that the drug in the blood plasma attaches to receptors or when it is stored temporarily in bone or fat (2).

 4. Inactivation

 This process, which is also known as biotransformation, is the way in which a drug becomes inactive "through a chemical change, usually by metabolic processes in the liver." (2)

 5. Excretion

 This refers to the way in which the drug is eliminated from the body through urine feces, saliva, and sweat (2). The most common one being urine.



Sources
1. http://www.merriam-webster.com/dictionary/bioavailability

2. "Psychopharmacology: Drugs, the Bran, and Behavior," by Jerrold S. Meyer and Linda F. Quenzer. 

3. http://www.merriam-webster.com/dictionary/pharmacokinetics

4. http://medical-dictionary.thefreedictionary.com/enteral

5. https://www.britannica.com/topic/parenteral-administration

6. https://www.nlm.nih.gov/medlineplus/ency/article/002383.htm

7. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3189662/

8. http://www.merriam-webster.com/dictionary/subcutaneous

9. "Physiology of Behavior" by Neil Carson

10, http://www.ncbi.nlm.nih.gov/pubmed/6126289

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