JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD Pharm. D I Year PHARMACEUTICS Previous Question Paper (September/October - 2021 - R08)

Posology is the branch of pharmacy that deals with the study of doses or the quantitative determination of medicinal agents to be administered to a patient to produce the desired therapeutic effect.

The selection of an appropriate dose is influenced by several physiological, pathological, and pharmacological factors:

• Age: Infants and elderly patients generally require smaller doses due to differences in metabolism, renal excretion, and body composition. Pediatric doses are often calculated based on age, weight, or body surface area using formulas like Young's or Dilling's rule.
• Body Weight: The average adult dose is typically based on a 70 kg body weight. For individuals significantly deviating from this, doses are adjusted proportionally.
• Sex: Women may require different dosages than men due to differences in body fat percentage, hormonal activity, and physiological variations. Special caution is required during pregnancy and lactation to avoid teratogenic effects or drug transfer to the infant.
• Pathological Conditions: Diseases affecting the liver (metabolism) or kidneys (excretion) necessitate dose reductions to prevent toxicity. Similarly, cardiovascular or gastrointestinal disorders can alter drug absorption and distribution.
• Route of Administration: Intravenous administration delivers the drug directly into the circulation, requiring smaller doses compared to oral administration, which is subject to the first-pass effect and variable absorption.
• Time of Administration: Certain drugs are more effective or less toxic when administered at specific times (e.g., hypnotics at night, irritants after meals).
• Drug Tolerance and Idiosyncrasy: Tolerance requires higher doses to achieve the same effect, whereas idiosyncrasy refers to an abnormal, unexpected reaction to a drug regardless of the dose.
• Drug Interactions: The presence of other medications can lead to synergistic, additive, or antagonistic effects, requiring dosage adjustments to maintain safety and efficacy.
• Environmental Factors: Factors such as climate and occupational exposure can influence the patient's physiological response to medication.

An emulsion is a thermodynamically unstable system consisting of two immiscible liquids, one dispersed as globules in the other. Stability is achieved by adding emulsifying agents to reduce interfacial tension and form a protective barrier. Instability in emulsions manifests through several physical processes:

1. Flocculation: The aggregation of dispersed droplets into loose clusters, which can often be redispersed by shaking.
2. Creaming: The upward (or downward) movement of droplets due to density differences between the phases, governed by Stokes' Law. It is a reversible process.
3. Coalescence and Breaking: The irreversible fusion of small droplets into larger ones, eventually leading to the complete separation of the oil and water phases (cracking).
4. Phase Inversion: A change in the type of emulsion, such as oil-in-water (o/w) becoming water-in-oil (w/o), often caused by temperature changes or electrolyte addition.

Evaluation of emulsion stability is essential to ensure shelf-life and therapeutic efficacy:

1. Microscopic Examination: Used to observe droplet size distribution and detect early signs of coalescence or flocculation.
2. Phase Separation Studies: Centrifugation is used to accelerate the creaming or cracking process to predict long-term stability under stress conditions.
3. Rheological Measurements: Viscosity is measured using viscometers or rheometers. A change in viscosity over time is a primary indicator of instability.
4. Particle Size Analysis: Techniques like laser diffraction or Coulter counter analysis track changes in mean droplet diameter, as growth in size indicates impending instability.
5. Accelerated Stability Testing: Emulsions are stored at elevated temperatures (e.g., 40°C) or subjected to freeze-thaw cycles to observe physical changes over a shorter timeframe.
6. Conductivity Measurements: Changes in electrical conductivity help identify phase inversion or the continuous phase in an emulsion.