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5. Adsorbents

Diluents

Inert substances added in sufficient quantity to increase the bulk of the tablet powder to a
practical size for compression.

Note:
It’s difficult to compress powders weighing less than 50 mg.
If the drug dose is large (e.g., aspirin 300 mg), there is no need for a diluent.

a) Organic Diluents

1. Lactose – Water-soluble, pleasant taste, neutral reaction, non-hygroscopic. Available as
crystalline and spray-dried forms (for direct compression).

2. Mannitol – Expensive; produces a cool sensation in the mouth when dissolving. Mainly
used in chewable tablets.

3. Dextrose & Sucrose – Usually combined with other diluents due to disadvantages
(hygroscopic and unsuitable for diabetics).

4. Starch – Used mainly as a disintegrant and glidant; not commonly as a diluent due to
poor compressibility (used more often in capsules). Modified corn starch improves
compressibility but dissolves poorly.

5. Microcrystalline Cellulose – Expensive, insoluble in water, excellent compressibility,
used mainly for direct compression.

b) Inorganic Diluents

1. Dicalcium phosphate

2. Calcium sulphate

o Both have good compressibility, are insoluble in water, non-hygroscopic, and
inexpensive.

Selection Criteria

1. Formulator experience and cost (should not be expensive).

2. Compatibility with drug and other excipients — diluent must be inert.

o Example:

▪ Ca salts + tetracycline → insoluble complex (poorly absorbed).

▪ Drugs with primary amine + carbonyl diluent (lactose, dextrose, sugar)
→ colored Schiff’s base (brown patches).

3. Initial moisture content and ability to absorb moisture — excessive moisture may cause
hydrolysis or physical instability.

o Moisture forms:

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