Therapeutic Action of Different Classes of Drugs
A high-yield master guide to Antacids, Analgesics, Antibiotics, and more.
To prescribe the right medicine, doctors and chemists classify drugs based on their Therapeutic Action—meaning what biological effect they have on the body to cure a specific ailment. Let's break down the major classes of pharmaceutical drugs you must know for competitive exams.
1. Antacids and Antihistamines
A. Antacids
Overproduction of acid in the stomach causes irritation and pain. Antacids neutralize this excess acid.
- Historical: Sodium bicarbonate ($NaHCO_3$) or Magnesium/Aluminum hydroxides were used. Problem: They only treat the symptom, and excess $NaHCO_3$ can actually trigger the stomach to produce more acid.
- Modern Breakthrough: Chemists discovered that the chemical Histamine stimulates the secretion of pepsin and $HCl$ in the stomach.
- Modern Antacids: Drugs like Cimetidine (Tegamet) and Ranitidine (Zantac) were designed to prevent histamine from interacting with receptors in the stomach wall, effectively stopping acid production at the source.
B. Antihistamines
Histamine is a potent vasodilator. It causes allergic reactions like nasal congestion, sneezing, and inflammation.
- Action: Antihistamines are drugs that interfere with the natural action of histamine by competing with histamine for binding sites of receptor where histamine exerts its effect.
- Examples: Brompheniramine (Dimetapp) and Terfenadine (Seldane).
2. Neurologically Active Drugs
These drugs affect the message transfer mechanism from nerve to receptor. They are broadly divided into Tranquilizers and Analgesics.
A. Tranquilizers
Used for the treatment of stress, and mild or even severe mental diseases. They induce a sense of well-being, relieving anxiety and tension.
If levels of noradrenaline (a mood-regulating neurotransmitter) are low, depression occurs. Antidepressants inhibit the enzymes that degrade noradrenaline. Examples: Iproniazid, Phenelzine (Nardil).
Used for relieving everyday tension. Examples: Chlordiazepoxide, Meprobamate. Equanil is used in controlling depression and hypertension.
Derivatives of barbituric acid. Examples: Veronal, Amytal, Nembutal, Luminal, Seconal. Others include Valium and Serotonin.
B. Analgesics (Pain Relievers)
Analgesics reduce or abolish pain without causing impairment of consciousness, mental confusion, or paralysis.
- Non-Narcotic (Non-Addictive) Analgesics:
Examples: Aspirin and Paracetamol.
Mechanism: Aspirin inhibits the synthesis of chemicals known as prostaglandins, which stimulate inflammation and cause pain in the tissues. Aspirin is also used to prevent heart attacks due to its anti-blood-clotting action. - Narcotic (Addictive) Analgesics:
Examples: Morphine, Heroin, Codeine.
Use: Used for severe pain (post-operative pain, cardiac pain, terminal cancer). When administered in medicinal doses, they relieve pain and produce sleep. In poisonous doses, they produce stupor, coma, convulsions, and death.
3. Antimicrobials
Diseases in humans are caused by a variety of microorganisms (bacteria, viruses, fungi). Antimicrobials tend to destroy/prevent the development of these microbes.
A. Antibiotics
Chemical substances produced completely or partially by chemical synthesis, which in low concentrations inhibit the growth or destroy microorganisms.
*Historical Note: Paul Ehrlich developed Salvarsan (Arsphenamine) to treat Syphilis. He later discovered Prontosil, which led to the discovery of Sulpha drugs (Sulphanilamide).*
- Penicillin
- Aminoglycosides
- Ofloxacin
- Erythromycin
- Tetracycline
- Chloramphenicol
• Broad spectrum: Kills a wide range of Gram-positive and Gram-negative bacteria (e.g., Chloramphenicol, Amoxicillin).
• Narrow spectrum: Effective against mainly Gram-positive or mainly Gram-negative bacteria (e.g., Penicillin G).
Figure 1: Concentration determines the therapeutic action. The same chemical can be an antiseptic or a toxic disinfectant.
B. Antiseptics
Applied to the living tissues such as wounds, cuts, ulcers, and diseased skin surfaces. They are not ingested like antibiotics.
- Dettol: A mixture of chloroxylenol and terpineol.
- Bithionol: Added to soaps to impart antiseptic properties.
- Tincture of Iodine: 2-3% solution of Iodine in alcohol-water mixture.
- Iodoform: Used as an antiseptic for wounds.
- Boric Acid: Weak antiseptic for eyes (in dilute aqueous solution).
C. Disinfectants
Applied to inanimate objects such as floors, drainage systems, instruments, etc. They are toxic to living tissues.
- Phenol: 1 percent solution is a powerful disinfectant.
- Chlorine: 0.2 to 0.4 ppm in aqueous solution is used to sterilize drinking water.
- Sulfur Dioxide ($SO_2$): In very low concentrations, it acts as a disinfectant.
4. Antifertility Drugs
These are chemical substances used to control the population by preventing pregnancy (Birth control pills). They essentially contain a mixture of synthetic estrogen and progesterone derivatives. Both of these compounds are hormones. Progesterone suppresses ovulation.
- Synthetic Progesterone derivatives: Generally more potent than natural progesterone. Example: Norethindrone.
- Synthetic Estrogen derivatives: Used in combination with progesterone derivatives. Example: Ethynylestradiol (Novestrol).
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