PHAR2911 · Pharmaceutics And Professional Practice
Capsules: Hard & Soft
This chapter covers the capsule as a solid oral dosage form: the hard capsule (a two-piece cap-and-body shell filled with powder or pellets) versus the soft capsule / softgel (a one-piece sealed shell holding a liquid or semisolid), plus shell chemistry, fill design and the capsule size ladder. It matters because PHAR2911 routinely tests the counter-intuitive size rule (a smaller number means a larger capsule), the gelatin moisture facts, and the dispensary excess + method-of-doubling compounding arithmetic — high-yield marks that are easy to bank if you know the canon cold.
What this chapter covers
- 01Hard (2-piece) vs soft (1-piece) capsule architecture
- 02Gelatin shell: collagen origin, 13-16% moisture as plasticiser
- 03Low-RH brittleness vs high-RH distortion (storage rule)
- 04HPMC and other shell alternatives (DPI / vegetarian)
- 05Capsule sizes: smaller number = larger capsule (000 to 5)
- 06Powder fill excipients and how the dose is released
- 07Enteric capsules and gelatin cross-linking trap
- 08Compounding: N+2 excess rule and method of doubling
Powder quantity for a compounded capsule prescription
- +1Apply the capsule excess rule: make for N + 2 capsules, so 20 prescribed + 2 extra = calculate for 22 capsules.
- +1Active required = 22 x 25 mg = 550 mg.
- +1Diluent (lactose) required = 22 x 175 mg = 3850 mg (3.85 g).
- +1Total blend = 550 + 3850 = 4400 mg; fill weight per capsule = 4400 / 22 = 200 mg.
- +1Blend by the method of doubling (geometric dilution): add an equal weight of lactose to the active, mix, then keep doubling the smaller portion until all diluent is in; fill only the 20 prescribed capsules, the 2-capsule excess covers mortar and tray losses.
Key terms
- Hard capsule
- A two-piece dosage form in which a pre-formed cap and body telescope together to enclose a dry fill (powder blend, pellets or mini-tablets); the second-most-common solid oral dosage form after the tablet.
- Soft capsule (softgel)
- A one-piece, hermetically sealed shell formed, filled and closed in a single operation (rotary-die process), used for liquids, oily solutions and semisolids dosed by volume.
- Gelatin
- The classic capsule shell material, a protein derived from collagen; edible, a good film-former and soluble at body temperature, with a bound moisture content of about 13-16% w/v that acts as a plasticiser.
- Capsule size number
- The fixed size designation for hard-shell capsules, running backwards so that a smaller number denotes a larger capsule (000 is the largest in common use at about 1.36 mL, down to size 5 at about 0.13 mL).
- Method of doubling
- A geometric-dilution mixing technique in which an active is blended with diluent by repeatedly doubling the smaller portion at each step, ensuring the drug is spread uniformly through the powder.
- Enteric capsule
- A capsule made from a pH-sensitive cellulosic shell that resists acid and opens in the small intestine, so it needs no separate enteric coating to protect an acid-labile drug or the stomach.
Capsules: Hard & Soft FAQ
Is a smaller capsule number bigger or smaller?
Bigger. The size system runs backwards: the smaller the number, the larger the capsule. Size 000 is the largest in common use (about 1.36 mL) and size 5 is the smallest (about 0.13 mL). When formulating you pick the smallest size that comfortably holds the fill, because smaller capsules are easier to swallow.
What is the difference between a hard capsule and a soft capsule?
A hard capsule is two pieces (a cap and a body that telescope together) and is filled with a powder, pellets or mini-tablets. A soft capsule, or softgel, is a single sealed piece made, filled and closed in one operation, and it holds a liquid, oily solution or semisolid dosed by volume.
Why is the gelatin shell sensitive to humidity?
The 13-16% w/v water bound in a gelatin shell acts as a plasticiser that keeps it flexible. At low relative humidity the shell dries out and becomes brittle, cracking or splitting; at high relative humidity it absorbs water, softens, distorts and sticks together. That is why capsules are stored in tightly closed containers away from humidity extremes.
When would you use an HPMC capsule instead of gelatin?
HPMC (hypromellose) keeps its strength at low humidity, so it is chosen for moisture-sensitive fills and for inhalation / dry-powder-inhaler (DPI) capsules that must stay crisp. It is also a vegetarian, animal-free option. Chitosan and modified starch are other plant-based alternatives.
How much extra powder do you weigh when compounding capsules?
Make an excess equivalent to 2 extra capsules: calculate every ingredient for N + 2 capsules, blend the powder by the method of doubling, then fill and supply only the N prescribed. The two extra capsules' worth of powder absorbs losses to the mortar, pestle and filling tray so each dispensed capsule is full.
Exam move
Memorise the capsule answer pattern and you can bank these marks fast. First, name the architecture in one line: hard = 2-piece, powder fill; soft = 1-piece, liquid fill. Second, lock the shell facts cold - gelatin from collagen, 13-16% moisture as plasticiser, brittle at low RH and distorts at high RH, HPMC for DPI/low-moisture - because these are pure recall. Third, never confuse the size ladder: drill "smaller number = bigger capsule" until it is automatic (000 biggest, 5 smallest), since the wording is deliberately counter-intuitive. Fourth, for any compounding calculation, write "make for N + 2" before touching numbers, multiply each ingredient by N+2, then state "blend by method of doubling, fill only N". Finally, keep capsule QC separate from tablet QC: capsules use uniformity of mass (<300 mg gives 10%, >=300 mg gives 7.5%) and content (85-115%), but have no hardness or friability test because there is no compressed compact.
