The term “pharmacology” is derived from two Greek words: pharmakon, the Greek word for drugs, and logos, the Greek word for science. It is a branch of science that deals with the study of substances that interact with living systems through chemical processes, especially by binding to regulatory molecules and activating or inhibiting normal body processes. These substances may be chemicals administered to achieve a beneficial therapeutic effect on some process within the patient or for their toxic effects on regulatory processes in parasites infecting the patient.
The history of pharmacology is as old as man himself. Early man undoubtedly recognized the medicinal or toxic effects of many plant and animal materials. Belief in the curative powers of these materials rested exclusively upon traditional knowledge, that is, empirical information not subjected to critical examination. These knowledge were passed down through generations. Whatever has been used from the earliest times in the form of superstition, primitive material medica, herbal, traditional, magic and witchcraft metamorphosed now into a highly organized science in its own right called pharmacology.
- 1 Branches of pharmacology
- 2 What are drugs?
- 3 Sources of drugs
- 4 Drug Nomenclature
- 5 Routes of drug administration
- 6 Effects of drugs
- 7 Why do the effects of drugs vary between different people?
- 8 References
Branches of pharmacology
Pharmacology has two major branches:
- Pharmacokinetics – This describes the activities or drug’s actions as it moves through the body. These activities include absorption, distribution, metabolism, and excretion of drugs. This branch of pharmacology is also concerned with a drug’s onset of action, peak concentration level, and duration of action.
- Pharmacodynamics – Pharmacodynamics deals with the study of molecular, biochemical, and physiological effects of drugs, including drug mechanism of action. In simplest terms, it can be described as what the drug does to the body.
Others branches of pharmacology include clinical pharmacology, neuropharmacology, psychopharmacology, pharmacotherapeutics, pharmacogenetics, pharmacogenomics, toxicology, chemotherapy, posology, pharmacoeconomics, pharmacoepidemiology, molecular pharmacology, immunopharmacology, pharmacometrics etc.
Read Also: Introduction to Pharmaceutical Microbiology
What are drugs?
The word “drug” is derived from the old French word “drogue” which means a “dry herb”. It is defined as any substance or product which when administered to a living organism, influences biological functions. Drugs can also be defined as any substance that is used or intended to be used to modify or explore physiological systems or pathological states for the benefit of the recipient.
The terms “drug” and “medicine” are often mistakenly used by most people (practitioners and laymen alike) without any marked distinction. However, in the strictest sense, the two have different meanings, and they can serve different purposes.
Sources of drugs
The major sources of drugs can be grouped into the following
a. Plant Sources
Many plants have been used for centuries as drugs or drug sources. These include leaves, barks, fruits, roots, stem, wood, seeds, blossoms, bulb etc. Drugs from plants may either be used without further processing (crude drugs) or with technical processing (prepared drugs).
Some pharmacologically active principles or drugs derived from plant sources include digoxin and digitoxin (from Digitalis purpurea/foxglove plant), atropine (from Atropa belladonna), quinine (from Cinchona), tubocurarine (from Chondrodendron tomentosum) etc.
b. Microbial sources
Several life-saving drugs have been derived from microorganisms. Examples include penicillin produced by Penicillium chrysogenum, streptomycin from Streptomyces griseus, chloramphenicol from Streptomyces venezuelae, neomycin from Streptomyces fradiae, bacitracin from Bacillus subtilis etc.
c. Animal Sources
Many important drugs are derived from animal source. In most instances, these medicinal substances are derived from the animal’s body secretions, fluid or glands. Animal sources include cod liver oil from Gadus spp., insulin from bovine or porcine pancreas, hirudin obtained from the European medical leech (Hirudo medicinalis), heparin from Mexican medical leech (Hirudo manillensis) etc. Like plant products, drugs from animal sources may be crude (unrefined) or refined material.
d. Marine sources
Coral, sponges, fish, and marine microorganisms produce biologically potent chemicals used in the prevention, treatment or cure of many diseases. A good example of such chemicals is Curacin A, a potent cytotoxic agent from marine cyanobacterium Lyngbya majuscule. Other examples include eleutherobin from coral Eleutherobia sp., discodermolide from marine sponge Discodermia dissoluta, etc.
e. Mineral sources
Chemically pure forms of both metallic and non-metallic minerals are useful pharmacotherapeutic agents. For examples, ferrous sulfate used in iron deficiency anaemia, magnesium sulphate used as purgative; magnesium trisilicate, aluminium hydroxide and sodium bicarbonate used as antacids for hyperacidity and peptic ulcer etc.
Radioactive isotopes of iodine, phosphorus, gold are increasingly becoming important in medicine both for diagnosis and treatment of diseases particularly malignant conditions.
f. Semi-synthetic Sources
Semi-synthetic drugs are generally made by chemically modifying substances that are available from natural source to improve its potency, efficacy and/or reduce side effects. In semi-synthetic drugs, the nucleus of drug obtained from natural source is kept intact but the chemical structure is altered.
Examples of semi-synthetic medicine include heroin from morphine, bromoscopolamine from scopolamine, ampicillin from penicillin etc.
g. Synthetic sources/chemical derivatives
Synthetic drugs are manufactured in pharmaceutical or chemical laboratories using chemical synthesis, which rearranges chemical derivatives to form a new compound. They may be either organic or inorganic or a combination of both.
At present, majority of drugs used in clinical practice are prepared synthetically. One of the earliest synthetic drugs was sulphonamide, which began with the synthesis of prontosil dye. Other examples include aspirin, acetaminophen, chloroquine etc.
h. Biosynthetic sources (genetically engineered drugs)
This is relatively a new field which is being developed by mixing discoveries from molecular biology, recombinant DNA technology, DNA alteration, gene splicing, immunology, and immune pharmacology. Examples include recombinant Hepatitis B vaccine, recombinant insulin and others.
Read More: Sources of Drugs
Throughout the process of development, drugs may have several names assigned to them. These names are the drug’s chemical name, generic name, and brand name.
i. Chemical name
This gives the exact chemical makeup of the drug and indicates the arrangement and position of atoms or atomic groups. A chemical name looks strange to anyone who is not a chemist and is difficult for most people to pronounce e.g., 1-Cyclopropyl-6-fluoro-4-oxo-7-(piperazin-1-yl)-1,4-dihydroquinoline-3-carboxylic acid Hydrochloride.
ii. Generic name
This is the name with which the drug is described in official books of reference like pharmacopoeias e.g., ciprofloxacin.
iii. Brand name
This is the name given to a drug by its manufacturer. This name is often followed by the symbol ® which indicates that the name is registered to a specific manufacturer and no one else can use that name. Examples include Ciloxan®, Cipro®, Neofloxin® etc.
Routes of drug administration
Drugs can be administered through several routes, which are sometimes referred to as transitory passages. The major routes of administration include:
- Oral route
- Sublingual/ Buccal route
- Rectal route
- Topical route
- Transdermal route
- Inhalational route/ pulmonary route
- Injection route e.g., subcutaneous (SC) injection, intramuscular (IM) injection, intradermal (ID) injection, intravenous (IV) injection, intra-arterial (IA) injection, intrathecal (IT) injection, intraperitoneal (IP) injection and intravitreal injection.
Read More on Routes of Drug Administration
Effects of drugs
Drugs have multiple effects on the body. The effect produced by drugs can be recognised only as a change in a function or process that maintain the existence of the living organism since all drugs act by causing changes in some known physiological functions and processes.
Some of these effects are desirable and some are not. The therapeutic effect is the intended physiological effect or the reason the drug is being given. This can be the drug’s action against a disease such as an antibiotic destroying or slowing down the growth of bacteria. Another physiological effect can be the side effects that occur in the body such as nausea and vomiting or a skin rash.
A side effect is a physiologic effect that is not the intended action such as the drowsiness that occurs when a patient takes first-generation antihistamine. Some side effects are beneficial while others are adverse effects that can be harmful to a patient.
Why do the effects of drugs vary between different people?
Some of the general factors that may influence drug effects include
Body weights of individuals affect drug therapy responses. In general, dosages are based on a weight of approximately 150 lb, which is calculated to be the “average” weight of men and women. For many drugs higher than average dosages may be necessary to produce the desired effect in an overweight individual. An underweight individual, on the other hand, may require a smaller dosage to produce the desired effect.
Ethnicity changes the expected effects of drugs in individuals. For example, many people of Asian descent have a greater-than-usual response to the anticoagulant drug warfarin, which greatly increases their risk for excessive bleeding, haemorrhage, and stroke. As a result, the starting dosage of warfarin is lower for Asians and increased at a slower rate until the individual patient’s response is known.
The age of the patient may influence the effects of a drug. For instance, infants and children require smaller doses of a drug than adults do. This is because they have smaller fat and total water content, immature enzyme systems, reduced kidney function, immature livers, and variation in circulating blood proteins which affects their ability to metabolize drugs.
Elderly patients may also require smaller dosages because of decline in liver size, blood flow, and enzyme production as well as the presence of several disease processes, and the necessity for many medications.
The gender of an individual may influence the action of some drugs. Women may require a smaller dose of some drugs than men. This is because many women are smaller than men and have a body fat-and-water ratio different from that of men.
There is great variation in the activity of cytochrome P-450 enzyme system from one person to the next as a result of genetic differences. Some persons may be rapid metabolizer of drugs and require higher-than-expected dosages to achieve the same desired effect. Others may be slow metabolizer and have more problems with side effects, adverse reactions, and toxic effects even from what are considered “normal” drug dosages.
vi. Disease state
The presence of certain diseases may influence the action of some drugs. Sometimes disease is an indication for not prescribing a drug or for reducing the dose and frequency of administration of certain drugs. In liver (hepatic) disease, for example, the ability to metabolize or detoxify a specific type of drug may be impaired. If the average or normal dose of the drug is given, the liver may be unable to metabolize the drug at a normal rate. Consequently, the drug may be excreted from the body at a much slower rate than normal.
Patients with kidney disease may exhibit drug toxicity and a longer duration of drug action. The dosage of drugs and frequency of administration may be reduced to prevent the accumulation of toxic levels in the blood or further injury to the kidney.
vii. Poly-drug use
Use of other prescribed, over-the-counter, or illicit drugs, as well as alcohol intake often increase the activity of metabolic enzyme systems. This change increases the rate at which some drugs are deactivated and eliminated, often requiring that the doses be increased and/or given more frequently to be effective. For example, opioid
drugs for pain are metabolized and eliminated much faster in the person who drinks alcohol on a daily basis. The dose then may need to be higher for the person to obtain pain relief.
- Aguwa, C. and Akah, P. (2006). How Drugs Act. In C. Aguwa and J. Ogbuokiri (Eds.), A Handbook of Pharmacology for Nursing and Allied Health Professions (pp. 2-7). Nigeria: Africana First Publishers Limited.
- Alamgir, A. (2017). Therapeutic Use of Medicinal Plants and Their Extracts: Volume 1. Switzerland: Springer International Publishing AG.
- Edmunds, M. (2016). Introduction to Clinical Pharmacology (8th ed.). USA: Mosby.
- Galbraith, A., Bullock, S., Manias, E., Hunt, B. and Richards, A. (2013). Fundamentals of Pharmacology: An Applied Approach for Nursing and Health (2nd ed.). USA: Routledge.
- Kamienski, M. and Keogh, J. (2006). Pharmacology Demystified. New York: McGraw-Hill Companies, Inc.
- Raj, G. and Raveendran, R. (2019). Introduction to Basics of Pharmacology and Toxicology Volume 1: General and Molecular Pharmacology: Principles of Drug Action. Singapore: Springer Nature Singapore Pte Ltd.
- Visovsky, C., Zambroski, C., Hosler, S. and Workman, L. (2019). Introduction to Clinical Pharmacology (9th). USA: Elsevier Inc.
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