Semi-solid Dosage Form | Definition, advantage, disadvantage, ingredients

Semi-solid means one substance which contains both solid and liquid. Semisolid dosage forms are also contain solid and liquid both. These types of dosage forms are viscous in nature. Normally used for topical or external application.

Definition of Semi-solid Dosage Form:

Semisolid dosage forms are the topical dosage form used for the therapeutic, protective or cosmetic functions. It may be applied to the skin, nasal, vaginal, or rectal cavity.

Examples of Semi-solid Dosage Form:

Ointments, pastes, cream, plasters, gels, and rigid foams.

Advantages of semi-solid dosage form:

• It is used externally
• The probability of side effects can be reduced
• First, pass gut and hepatic metabolism is avoided.
• Local action and Site-specific action of the drug on the affected area.
• Convenient for unconscious patients or patients to have difficulty in oral administration.
• Suitable dosage form for bitter drugs.
• More stable than a liquid dosage form

Disadvantages of semi-solid dosage form:

• The accuracy can't be measured, for the semisolid dosage form.  
• May cause staining.
• They are bulky to handle.
• Application with a finger may cause contamination.
• Physico-chemical is less stable than a solid dosage form.
• May cause irritation or allergy in some patients

Ideal properties of semisolid dosage forms:

• Smooth texture
• Elegant in appearance
• Non-dehydrating
• Non-gritty
• Non-greasy and non-staining
• Non-hygroscopic

Related Article: 
Solid Dosage Form | Definition, Types, Excipient, and more.
Liquid dosage form | Definition, classification, advantage, disadvantage, and more.

Ingredients needed for semisolid dosage form:

• Bases
• Preservative
• Humectants
• Antioxidants
• Emulsifier
• Gelling agent
• Permeation enhancer
• Buffers

Bases used in semisolid dosage form:

It is one of the most important ingredients used in the formulation of the semisolid dosage form. Ointment bases do not merely act as the carriers of the medicaments, but they also control the extent of absorption of medicaments incorporated in them.

Ideal characterization of bases used in semisolid dosage form:

They should be:

1. Inert, non-irritating, and non-sensitizing.
2. Compatible with skin pH and the drug.
3. Good solvent and/or emulsifying agent.
4. Emollient, protective, non-greasy and easily removable.
5. Release medicament readily at the site of application.
6. Pharmaceutically elegant and possess good stability

Classification of Bases:

According to USP ointment bases are classified into four general groups:

• Hydrocarbon bases (oleaginous bases) (Petrolatum, Paraffin, Lanolin, etc.)
• Absorption bases (cold cream, anhydrous lanolin, etc.)
• Water-removable bases ( oil in water)
• Water-soluble bases (polyethylene glycol)

Antioxidants used in semisolid dosage form:

Oxygen has the capability to become a part of potentially damaging molecules called “free radicals.” Free radicals can attack the healthy cells of the body. It can break their structure and that's why the function is also affected.
Example: Butylated hydroxyanisole, Butylated hydroxytoluene

Ingredients that increase permeation:

• Skin can act as a barrier. There are various penetration enhancers that can help the drug to penetrate through the skin.
• Oleic acid

The emulsifier used in semisolid dosage form:

Emulsifiers are used to improve the stability of an emulsion by increasing its kinetic stability. It reduces surface tension and prevents coalescence. It helps to increase the viscosity at low concentration.

Emulsifying agents used in semisolid dosage form:

Sodium lauryl sulfate: Oil/Water emulsion - Sodium stearate and calcium stearate.
Glyceryl monostearate: This is a weak Water/Oil emulsifying agent and it is used as a stabilizer and emollient in the Oil/Water emulsion.

Humectant used in semisolid dosage form:

The humectant is a hygroscopic substance used to increase the solubility of the active ingredient to increase skin penetration. It's also used to improve the hydration of the skin.

Buffers used in semisolid dosage form:

Buffers are added for various purposes. Such as
(i) Compatibility with skin.
(ii)Drug solubility.
(iii)Drug stability.
(iv)Influence the ionization of drugs.

Preservatives used in semisolid dosage form:

Preservatives are used to inhibit the growth of contamination of microorganisms.
Example: Para-hydroxybenzoate (parabens), phenols, benzoic acid, sorbic acid, etc.