Questions
3–4 questions in university semester exams
Difficulty
Medium
Importance
High yield for pharmacology and clinical nursing modules
Overview
Pharmacokinetics describes the body's influence on drugs through absorption, distribution, metabolism, and excretion (ADME). Understanding these principles alongside the mechanism of analgesics is vital for clinical safety and exam success, as they form the foundational basis for drug dosage calculations.
Routes of Drug Administration
The route of administration determines the speed of onset and the bioavailability of a drug. Selection depends on the patient's state, the drug's properties, and the desired site of action.
- Enteral routes: Oral, sublingual, rectal
- Parenteral routes: Intravenous (100% bioavailability), intramuscular, subcutaneous
- Topical: Transdermal patches for systemic absorption
- Inhalation: Rapid absorption via pulmonary capillaries
Drug Absorption & Half-Life
Absorption is the movement of a drug into the bloodstream, while half-life (t1/2) is the time required for the concentration of a drug in the plasma to decrease by 50%. Mastery of these concepts is essential for determining dosing intervals.
- Bioavailability (F): Fraction of drug reaching systemic circulation
- First-pass metabolism: Reduces bioavailability of oral drugs
- Half-life formula: t1/2 = 0.693 / k (elimination rate constant)
- Steady state: Usually achieved after 4-5 half-lives
NSAIDs & Analgesics
Analgesics are classified into non-opioids (NSAIDs) and opioids. NSAIDs primarily act by inhibiting the COX-1 and COX-2 enzymes, thereby reducing prostaglandin synthesis which mediates pain and inflammation.
- Aspirin: Irreversible COX inhibitor, antiplatelet effect
- Paracetamol: Analgesic and antipyretic, weak anti-inflammatory
- Ibuprofen/Diclofenac: Non-selective COX inhibitors
- Celecoxib: Selective COX-2 inhibitor, lower GI risk
Formula Sheet
t1/2 = 0.693 / ke
Loading Dose = (Target Concentration * Volume of Distribution) / Bioavailability
Exam Tip
Always draw a labeled graph of plasma concentration versus time when asked about half-life to visualize the steady state and elimination phases clearly.
Common Mistakes
- Confusing the duration of action with the drug's half-life
- Failing to mention the COX-2 selectivity difference between various NSAIDs
- Assuming all parenteral routes have 100% bioavailability
More Revision Notes
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