A very broad definition of a drug would include "all chemicals other than food that affect living processes." A more concise definition of a drug is any substance that has an effect when ingested or introduced into the body. If the effect helps the body, the drug is a medicine. If a drug causes a harmful effect on the body, the drug is a poison. Any drug can be a medicine and a poison depending on its concentration (dose) and the metabolic conditions of the person taking the drug.
It is important to distinguish between actions of drugs and their effects. Actions of drugs are the biochemical physiological mechanisms by which the chemical produces a response in living organisms. The effect is the observable consequence of a drug action. For example, the action of penicillin is to interfere with cell wall synthesis in bacteria and the effect is the death of the bacteria.
One major problem of pharmacology is that no drug produces a single effect. The primary effect is the desired (beneficial) therapeutic effect. Secondary effects are all other effects besides the desired effect which may be either beneficial or harmful. The key is to regulate the drug dose to maximize the beneficial effects and minimize the non-desired side effects.
The biological effects observed after a drug has been administered are the result of an interaction between that chemical and some part of the organism. Mechanisms of drug action can be viewed from different perspectives, namely, the site of action and the general nature of the drug-cell interaction.
Drugs can be classified according to various criteria including chemical structure or pharmacological action. The preferred classification is the latter one which may be divided into main groups as follows:
- Chemotherapeutic agents - used to cure infectious diseases and cancer. (Sulfa drugs, Antibiotics)
- Pharmacodynamic agents - used in non-infectious diseases (Cholinergic, Adrenergic, Hallucinogenic, Sedatives)
- Miscellaneous agents (Narcotic Analgesics, Local Anesthetics)
Unfortunately, there is no connection between chemical structure and pharmacological activity. Drugs with similar chemical structure might have very different pharmacological activity. For example, cortisol is a steroid drug used to regulate the body's anti-inflammatory processes. Ibuprofen is another drug used in anti-inflammatory processes. However, both drugs have very different chemical structures.
Cortisol, a steroid drug used to regulate the body's anti-inflammatory processes. Image by NEUROtiker, Public domain, via Wikimedia Commons
Ibuprofen, a non-steroid drug used to regulate the body's anti-inflammatory processes. Image by Benjah-bmm27, Public domain, via Wikimedia Commons.
Drugs have three or more names including a: chemical name, brand or trade name, and generic or common name. The chemical name is assigned according to rules of nomenclature of chemical compounds. The brand name is always capitalized and is selected by the manufacturer. The generic name refers to a common established name irrespective of its manufacturer.
In most cases, a drug bearing a generic name is equivalent to the same drug with a brand name. However, this equivalency is not always true. Although drugs are chemically equivalent, different manufacturing processes may cause differences in pharmacological action. Several differences may be crystal size or form, isomers, crystal hydration, purity-(type and number of impurities), vehicles, binders, coatings, dissolution rate, and storage stability.
Mode of Drug Action
Drugs act within the cell by modifying normal biochemical reactions. There are three main mechanisms of drug action:
Drug-Receptor Inhibition (antagonist drugs)
Drugs can act within the cell by blocking the substrate binding to an enzyme or receptor. Because these drugs prevent an enzyme from performing its function, the drug is called an antagonist. Enzyme inhibition may be reversible or nonreversible; competitive or non-competitive. For example, the antibiotic drug sulfanilamide competitively binds to the enzyme in the dihydropteroate synthase (DHPS) active site by mimicking the substrate para-aminobenzoic acid (PABA). This prevents the substrate itself from binding which halts the production of folic acid, an essential nutrient for bacteria.
Drug-Receptor Activation (agonist drugs)
Drugs can act within the cell by binding to specific receptors located on the cell membrane. The binding of the drug to the receptor triggers an intracellular response. Some receptor sites have been identified with specific parts of proteins and nucleic acids. In most cases, the chemical nature of the receptor site remains obscure. For example, the drug Morphine binds to opioid receptors (inhibitory G protein-coupled proteins). After binding to its receptors, an internal cellular response is triggered. Activation of the inhibitory G protein-coupled proteins produces analgesia and respiratory depression.
Some drugs act exclusively by physical means outside of cells, without involving a drug-receptor interaction. These extracellular sites include external surfaces of the skin and gastrointestinal tract. Neutralization of stomach acid by antacids is a good example.
Charles Ophardt (Professor Emeritus, Elmhurst College); Virtual Chembook