19.1 Dosage Form Design & the Biopharmaceutics Classification System

Dosage Forms and Why Route Choice Matters

A dosage form is the physical vehicle that delivers a drug to the body in a stable, measurable, and clinically appropriate way. The same drug substance can be formulated into several dosage forms, and the choice of form plus route of administration governs three things the exam tests repeatedly: onset of action, extent of bioavailability, and chemical stability.

Solid oral dosage forms

Tablets and capsules are the most common solid oral forms because they are convenient, dose-accurate, and stable. Modified-release tablets manipulate where or how fast the drug releases:

• Enteric-coated tablets (e.g. aspirin low-dose, omeprazole) resist gastric acid and dissolve in the higher pH of the small intestine, protecting either the drug from acid degradation or the stomach mucosa from irritation.
• Sustained-release / extended-release formulations (e.g. metoprolol succinate ER, morphine SR) release drug slowly to maintain therapeutic plasma concentrations over 12-24 hours, smoothing peaks that cause side effects and troughs that allow symptom breakthrough.

Liquid dosage forms

• Solutions — drug fully dissolved; fastest oral onset among liquids because no dissolution step is required.
• Suspensions — insoluble drug dispersed in a liquid vehicle; must be shaken before dosing and may sediment on storage.
• Emulsions — two immiscible phases stabilised by an emulsifying agent; prone to creaming and cracking.
• Syrups — concentrated sucrose solutions that improve taste and act as a mild preservative.
• Elixirs — hydroalcoholic solutions (e.g. paracetamol elixir); alcohol co-solvent improves solubility of poorly water-soluble drugs.
• Tinctures — alcoholic extracts of botanical or drug material.

Semisolid, parenteral, and specialised forms

Exam trap: Enteric-coated and sustained-release tablets must never be crushed or split — doing so destroys the modified-release mechanism, causing either gastric irritation (enteric coat lost) or dose dumping with potential toxicity (sustained-release matrix broken). If a patient cannot swallow intact, an alternative formulation (liquid, dispersible, or IV) must be chosen.

The Biopharmaceutics Classification System (BCS)

The BCS is a regulatory framework that classifies oral drug substances by two properties: solubility and intestinal permeability. A drug is considered highly soluble when its highest therapeutic strength dose dissolves in 250 mL of aqueous media across the pH range 1-7.5 at 37 °C. High permeability generally means ≥90% absorbed in humans. The four classes predict the rate-limiting step for oral absorption:

Why this matters clinically and on the exam:

• Class II drugs benefit from formulation strategies that increase dissolution rate — micronisation (reducing particle size to increase surface area), salt formation, or amorphous solid dispersions.
• Class III drugs are already soluble, so making them smaller or saltier does not materially improve absorption; instead, absorption enhancers or prodrug approaches that increase permeability are needed.
• Class I and III drugs (highly soluble) can use dissolution testing as a bioequivalence surrogate in lieu of full in-vivo pharmacokinetic studies — the basis of the FDA biowaiver.

Biopharmaceutics: From Tablet to Systemic Circulation

Biopharmaceutics studies how the physicochemical properties of the drug and the dosage form, together with the route of administration, determine the rate and extent of drug absorption. For an immediate-release oral tablet, absorption proceeds through three sequential steps:

1. Disintegration — the tablet breaks apart into primary drug particles under the action of disintegrants and gastrointestinal motility.
2. Dissolution — drug particles dissolve into the gastrointestinal fluid; governed by the Noyes-Whitney equation (covered in section 19.2).
3. Absorption — dissolved drug crosses the enterocyte membrane into the portal blood.

The slowest of these three steps is the rate-limiting step. For Class II drugs, dissolution is slowest; for Class III drugs, permeation across the membrane is slowest.

Bioavailability (F) is the fraction of the administered dose that reaches the systemic circulation in an unchanged, active form. For an IV dose, F = 1 (100%) by definition because the drug is placed directly into the bloodstream. For oral dosing, F is reduced by two factors: incomplete absorption across the gut wall and first-pass (presystemic) metabolism in the liver and gut wall. Drugs with high hepatic extraction ratios — propranolol, morphine, glyceryl trinitrate, levodopa — have markedly reduced oral bioavailability and may require much larger oral doses or non-oral routes (sublingual, transdermal, IV) to achieve a systemic effect.

Worked example: Morphine has an oral bioavailability of roughly 25-30% due to extensive first-pass metabolism. To achieve equivalent systemic exposure, a 10 mg IV dose would require approximately 30-40 mg orally — a clinical fact the DHA exam may frame as a dosing-conversion question.