Pharmaceutics: Dosage Form Design, Stability and Bioavailability
Core concepts
Pharmaceutics questions test whether you can link formulation science to patient outcomes. You must be fluent in preformulation (solubility, pKa, partition coefficient, polymorphism, hygroscopicity), rheology of suspensions and semisolids, micromeritics (particle size, flow, angle of repose), surface and interfacial phenomena, and the biopharmaceutical classification system (BCS). Know the difference between bioavailability (rate and extent reaching systemic circulation) and bioequivalence (comparability of two products within accepted limits of AUC and Cmax). Understand dissolution testing conditions, sink conditions, and the Noyes–Whitney relationship, plus the stability logic of reaction order, Arrhenius extrapolation, and shelf-life assignment (t90).
Traps examiners use
- Confusing zero-order (constant amount per time, e.g. sustained-release, some transdermal systems) with first-order (constant fraction per time) release; half-life is meaningful only for first-order.
- Mixing sterility with pyrogen-free; autoclaving does not destroy pyrogens.
- Forgetting that polymorph selection changes dissolution rate and can invalidate a licence.
- Assuming a higher drug dose always means higher bioavailability.
Worked example
A first-order degradation has k = 0.05 month⁻¹. Shelf life to 90% remaining: t90 = 0.105/k = 0.105/0.05 ≈ 2.1 months. If the same product degrades by zero order at 2% per month, t90 = 10/2 = 5 months. Identify the order first, then choose the equation - this single decision resolves most calculation items.