Pharmacology
Pharmacology is the study of drugs, which comes from the Greek terms Pharmacon (drug) and logos (discourse). It is generally concerned with interacting externally delivered chemical compounds with living systems. A drug is a chemical compound that can cause a biological response. Pharmacology comprises all elements of pharmacological knowledge, focusing on their efficacy and safety for medicinal use.
Drug
A drug is an active chemical in medicine used to diagnose, prevent, treat, or cure diseases, excluding contraceptives or health enhancers. It modifies or explores physiological systems or pathological states for the recipient’s benefit. While “drug” is often misused to mean addictive or illicit substances, it should primarily denote therapeutic or diagnostic agents.
Pharmacotherapeutics
Pharmacotherapeutics involves applying pharmacological knowledge alongside an understanding of diseases to prevent, mitigate, or cure them. It includes selecting the most suitable drug, determining the appropriate dosage, and deciding the duration of treatment, all tailored to the specific needs of the patient.
Clinical pharmacology
Clinical pharmacology is the study of drugs in humans, covering pharmacodynamics, pharmacokinetics, efficacy, safety, and adverse effects. It aims to optimize drug use and support evidence-based medicine.
Chemotherapy
Chemotherapy involves treating infections or malignancies with drugs that target the infecting organism or malignant cells while sparing host cells. Drugs are categorized as:
- Pharmacodynamic agents: Affect the recipient directly.
- Chemotherapeutic agents: Target parasites or malignant cells with minimal impact on the host.
Toxicology
Toxicology studies the harmful effects of drugs and chemicals, focusing on the detection, prevention, and treatment of poisonings. It also examines drug side effects, as the same substance can act as a drug or poison based on the dose.
Receptor
A macromolecule or binding site on or inside a cell that recognizes a signal molecule or drug and initiates a response without performing any other function.
Ligand
A molecule that binds selectively to specific receptors or sites, indicating affinity without implying functional change (e.g., both agonists and antagonists are ligands).
Agonist
An agent that activates a receptor to produce an effect similar to the physiological signal molecule.
Inverse Agonist:
An agent that activates a receptor to produce an effect opposite to that of an agonist.
Antagonist
An agent that blocks the action of an agonist on a receptor or its response without producing any effect on its own.
Partial Agonist:
An agent that activates a receptor to produce a submaximal effect while antagonizing the action of a full agonist.
Drug Abuse
Self-medication with a drug in a manner or amount deviating from approved patterns, often for continuous or occasional use, leading to social disapproval or regulatory concerns.
Drug Addiction
Compulsive drug use where procuring and using the drug takes priority over other activities, often with a tendency to relapse after withdrawal, even without physical dependence.
Intolerance
A toxic reaction to a drug at therapeutic doses due to a low individual threshold, seen in rare cases with specific drugs like triflupromazine or chloroquine.
Idiosyncrasy
A genetically determined abnormal drug response unique to certain individuals, producing uncharacteristic effects, such as excitement from barbiturates or aplastic anemia from chloramphenicol.
Drug Allergy
An immunologically mediated reaction unrelated to the drug’s action, occurring in sensitized individuals, affecting organs like skin, airways, and blood vessels.
Tachyphylaxis
Rapid tolerance development with repeated quick doses of certain drugs (e.g., ephedrine, nicotine) due to depleted stores or receptor desensitization.
Drug Dependence
A state where drug use for personal satisfaction is prioritized over basic needs, often despite known health risks, involving psychological and/or physical dependence.
Psychological Dependence
A belief that well-being is only achieved through drug use, causing emotional distress if the drug is unavailable, ranging from desire to compulsive craving.
Physical Dependence
A physiological state requiring the drug to maintain equilibrium, with withdrawal causing characteristic symptoms, as seen with opioids, barbiturates, or alcohol.
ESSENTIAL MEDICINES CONCEPT
The WHO defines Essential Medicines as drugs that meet the priority healthcare needs of the population. They are selected based on public health relevance, efficacy, safety, and cost-effectiveness. These medicines should be consistently available in adequate amounts, appropriate forms, assured quality, and at an affordable price.
Prescription and non-prescription drugs
Under drug rules, most drugs, including all antibiotics, are sold only with a prescription from a registered medical practitioner and are listed under Schedule H of the Drugs and Cosmetic Rules (1945). However, some drugs like simple analgesics, antacids, laxatives, vitamins, and ferrous salts are deemed safe and can be purchased without a prescription. These are called non-prescription or over-the-counter (OTC) drugs and are available even in grocery stores.
Orphan Drugs
Orphan drugs are medicines or biological products used to diagnose, treat, or prevent rare diseases or conditions, or common diseases in resource-poor countries, where the cost of development and marketing is unlikely to be recovered from sales. Examples include sodium nitrite, fomepizole, and liposomal amphotericin B. While life-saving, these drugs are commercially challenging to produce, so governments offer incentives like tax benefits to encourage their development (e.g., the Orphan Drug Act in the USA).
Routes of drug administration
Local routes
Local routes treat localized lesions at accessible sites with minimal or no systemic absorption. This allows high drug concentrations at the target site with minimal systemic side effects. These routes can also serve for systemic drug delivery (e.g., glyceryl trinitrate via skin ointment or transdermal patch) if absorption occurs.
- Topical application involves applying drugs externally for localized action, offering convenience and patient comfort. Drugs can be delivered to the skin, mucosa, eyes, ears, anal canal, or vagina as lotions, ointments, creams, sprays, drops, or suppositories. Nonabsorbable oral drugs (e.g., sucralfate), inhaled drugs for bronchi (e.g., salbutamol), and solutions or gels for the urethra (e.g., povidone-iodine) are also forms of topical therapy.
- Deeper tissues can be targeted using a syringe and needle for slow systemic absorption. Examples include intra-articular injections (e.g., hydrocortisone acetate in the knee joint), nerve infiltration, intrathecal injections (e.g., lidocaine), and retrobulbar injections (e.g., hydrocortisone acetate behind the eye).
- Arterial supply: Intra-arterial injections are used for angiographic contrast media or to localize anticancer drug effects in limb malignancies via the femoral or brachial artery.
Systemic Routes
Systemic routes deliver drugs into the bloodstream for distribution throughout the body, including the site of action.
- Oral ingestion is the oldest, safest, and most convenient method of drug administration, requiring no assistance and being cost-effective. Both solid forms (e.g., tablets, capsules) and liquids (e.g., syrups, elixirs) can be administered this way.
- Sublingual or buccal: In sublingual administration, the drug is placed under the tongue or on the buccal mucosa for rapid absorption, bypassing the liver and avoiding first-pass metabolism. Examples include GTN, buprenorphine, and desamino-oxytocin.
- Rectal administration uses suppositories or enemas for systemic effects, useful for patients with recurrent vomiting or unconsciousness. Though inconvenient, drugs like diazepam, paracetamol, and indomethacin can be absorbed with partial liver bypass through external hemorrhoidal veins.
- Cutaneous administration involves applying lipid-soluble drugs to the skin for slow, prolonged absorption, bypassing the liver. Absorption can be enhanced with rubbing, oily bases, or occlusive dressings.
- Inhalation: Volatile liquids and gases are administered by inhalation for rapid systemic action, as absorption occurs quickly through the alveoli. Controlled administration allows precise adjustments, but irritant vapors like ether may inflame the respiratory tract and increase secretions.
- Nasal: The nasal mucosa absorbs certain drugs like GnRH agonists and desmopressin, bypassing digestive juices and the liver. This route is also being explored for peptides like insulin and for bypassing the blood-brain barrier.
- Parenteral administration involves injecting drugs directly into tissue or blood, bypassing the gastrointestinal system for faster, more reliable action, useful in emergencies. While it avoids gastric irritation and food interference, it is more costly, invasive, and carries risks like tissue injury. The important parenteral routes are:
(i) Subcutaneous (s.c.)
The drug is injected into loose subcutaneous tissue, allowing slower absorption than intramuscular, and is suitable for small volumes and self-injection.
(ii) Intramuscular (i.m.)
The drug is injected into large skeletal muscles, offering faster absorption, mild irritant tolerance, and suitability for depot preparations but not for self-injection.
(iii) Intravenous (i.v.)
The drug is injected directly into veins for immediate effects, allowing irritant drugs but with risks like thrombophlebitis and tissue necrosis if extravasation occurs.
(iv) Intradermal injection
The drug is injected into the skin, raising a bleb or through multiple punctures, used for vaccines or sensitivity testing.
References
Latest Editions of
- Rang H. P., Dale M. M., Ritter J. M., Flower R. J., Rang and Dale’s Pharmacology,.Churchil Livingstone Elsevier
- K.D.Tripathi. Essentials of Medical Pharmacology, JAYPEE Brothers Medical Publishers (P) Ltd, New Delhi.
- Sharma H. L., Sharma K. K., Principles of Pharmacology, Paras medical publisher