Definition: Pastes are semisolid dosage forms intended for external application on the skin or mucous membranes. They are characterized by their high content of finely powdered solid materials (20–50%) dispersed in a suitable base, resulting in a stiff consistency. The high solid content differentiates pastes from ointments and creams, making them less greasy and more absorptive.
Characteristics of Pastes
1. High Solid Content: Contains a higher proportion of insoluble powders than ointments, which gives them a thicker consistency.
2. Non-Greasy: Pastes are less greasy due to their absorptive properties.
3. Protective: Provides a physical barrier on the skin, protecting against external irritants.
4. Adherence: Stays longer on the application site compared to ointments or creams.
5. Absorptive: Absorbs exudates, making them suitable for weeping or oozing lesions.
Components of Pastes
1. Active Ingredients: Therapeutic agents incorporated into the paste.
Examples: Zinc oxide, salicylic acid, coal tar.
2. Solid Powders: Provides the stiff consistency and absorptive properties.
Examples: Talc, starch, kaolin, titanium dioxide.
3. Base: Acts as a vehicle for the paste.
Examples: Petrolatum, lanolin, or aqueous gels.
4. Additives (Optional): Preservatives, antioxidants, or stabilizers for enhanced shelf life.
Examples: Methylparaben, butylated hydroxytoluene (BHT).
Types of Pastes
1. Fatty Pastes: Composed of powders dispersed in a fatty base like petroleum jelly.
Examples: Zinc oxide paste for diaper rash.
2. Aqueous Pastes: Contain water as the continuous phase with powders dispersed in it.
Examples: CMC-based pastes for oral ulcers.
Advantages of Pastes
1. Non-Greasy: Suitable for patients who dislike greasy formulations.
2. Protective Barrier: Shields the skin from external irritants and prevents water loss.
3. Reduced Skin Irritation: The high solid content reduces the irritant effects of certain active ingredients.
4. Prolonged Action: Sticks well to the skin, allowing for extended contact time.
5. Absorptive: Effective in managing moist lesions by absorbing fluids.
Disadvantages of Pastes
1. Stiff Consistency: May be difficult to apply and spread on the skin.
2. Residue: Leaves a thick residue on the skin that can be hard to remove.
3. Reduced Cosmetic Appeal: May appear chalky or unsightly.
4. Limited Use on Hairy Areas: Not suitable for application on areas with dense hair due to its thick consistency.
Applications of Pastes
1. Dermatological Treatments: Used to treat skin conditions like psoriasis, eczema, and diaper rash.
Example: Zinc oxide paste for soothing irritated skin.
2. Anti-Inflammatory Pastes:
Example: Diclofenac paste for localized pain relief.
3. Antifungal and Antibacterial Pastes:
Example: Whitfield’s ointment (salicylic acid and benzoic acid) for fungal infections.
4. Protective Applications:
Provides a barrier against moisture and irritants.
Example: Pastes used for pressure sores or to protect stoma sites.
5. Oral Pastes:
Used for the treatment of oral conditions such as ulcers or lichen planus.
Example: Triamcinolone acetonide paste.
Preparation of Pastes
1. Mechanical Incorporation: The powdered solid is gradually mixed with the base using geometric dilution to ensure even distribution.
2. Fusion Method: For fatty pastes, the base is melted, and the solid powders are incorporated with continuous stirring.
3. Specialized Techniques: High-shear mixing or planetary mixers for industrial-scale preparation.
Evaluation of Pastes
1. Physical Appearance: Color, texture, and homogeneity are assessed visually.
2. Spreadability: Determines ease of application and coverage on the skin.
3. Adhesion Test: Evaluates how well the paste adheres to the skin for prolonged action.
4. Drug Content Uniformity: Ensures even distribution of active ingredients.
5. pH Measurement: Ensures compatibility with skin or mucosal tissues.
6. Absorptive Capacity: Measured to assess the paste’s ability to absorb moisture or exudates.
Examples of Common Pastes
1. Zinc Oxide Paste: For diaper rash and minor skin irritations.
2. Salicylic Acid Paste: For hyperkeratotic conditions like warts or corns.
3. Coal Tar Paste: Used in psoriasis and eczema treatment.
4. Triamcinolone Oral Paste: For oral lichen planus or aphthous ulcers.
Pastes are versatile semisolid formulations that provide excellent protective, absorptive, and therapeutic properties. Their stiff consistency makes them ideal for conditions requiring a protective barrier or prolonged drug retention on the skin. Despite some limitations, pastes remain indispensable in dermatological and oral treatments.
