Pharmaceutics · Short notes

Dosage Form Design: Short Notes & Cheat Sheet

Preformulation, routes of administration, excipients and how a drug's properties decide its dosage form.

Short notes

Read these before you attempt the topic's MCQs.

A good dosage form balances the drug's properties, the patient and the route. Preformulation data comes first.

A dosage form delivers an accurate dose, protects the drug, masks taste, controls release and makes the drug convenient to take.
The choice depends on the drug (solubility, stability, dose), the patient (age, ability to swallow) and the route (oral, parenteral, topical, inhaled, rectal).
Oral solids are the most common: they are stable, accurate, cheap and easy to take.
Parenterals are chosen when a drug is destroyed in the gut (insulin), needs a fast onset, or the patient cannot swallow.

Preformulation characterises the pure drug before a formula is designed.
Key properties: solubility, pKa, partition coefficient (log P), particle size, polymorphism, hygroscopicity and stability.
Polymorphs are different crystal forms of the same drug. They differ in solubility and bioavailability; the metastable form dissolves faster than the stable form.
Amorphous solids dissolve faster than crystalline ones but are less stable.
Salts are made to improve solubility (e.g. sodium or hydrochloride salts of weak acids and bases).
Drug–excipient compatibility is checked early, e.g. lactose reacts with primary amines (Maillard reaction).

Excipients are the 'inactive' ingredients that make the dosage form work: diluents, binders, disintegrants, lubricants, glidants, preservatives, colours and flavours.
Excipients can still cause effects: lactose intolerance, benzyl alcohol toxicity in neonates, and allergy to certain dyes such as tartrazine.
Paediatric forms need suitable taste, small volume and avoidance of harmful excipients.

Oral: convenient, but subject to first-pass metabolism and variable absorption.
Sublingual and buccal: rapid onset and bypass first-pass (e.g. glyceryl trinitrate).
IV: 100% bioavailability and immediate effect, but cannot be recalled once given.
Transdermal: steady levels for days and bypasses first-pass (fentanyl, nicotine patches); only suits potent, lipophilic, low-dose drugs.
Inhaled: particles of about 1–5 µm reach the lower airways; larger ones deposit in the throat.

High-yield facts in one place. Cover the right-hand column and test yourself.

Excipient class	Examples
Diluent (filler)	Lactose, microcrystalline cellulose, dicalcium phosphate
Binder	Starch paste, PVP, HPMC, acacia
Disintegrant	Starch, croscarmellose, sodium starch glycolate, crospovidone
Lubricant	Magnesium stearate, talc, stearic acid
Glidant	Colloidal silicon dioxide, talc
Preservative	Parabens, benzalkonium chloride, benzyl alcohol
Antioxidant	Ascorbic acid, BHA, BHT, sodium metabisulfite

Route	Remember
Intravenous	Bioavailability 100%
Sublingual	Bypasses first-pass; fast onset
Transdermal	Potent, lipophilic, low-dose drugs only
Inhalation	1–5 µm particles reach the alveoli
Rectal	Partly bypasses first-pass; useful when vomiting
Intramuscular	Depot possible (oily or suspension injections)

Disintegrants 'Dive in' water: Disintegrants swell or wick water in to break the tablet apart; lubricants do the opposite (they are water-repelling).

The metastable polymorph is more soluble than the stable polymorph, not less.
Magnesium stearate is hydrophobic: too much of it slows disintegration and dissolution.
Excipients are not always inert; tartrazine and benzyl alcohol are classic examples.
Particles larger than about 10 µm deposit in the throat when inhaled.

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