Semi-solid dosage forms, also known as Quasi-solid, are highly viscous in nature, and are slightly flexible as liquids. A dosage form that is neither a complete solid nor a liquid and lies somewhere in between these forms is called a semi-solid dosage form. Ointments, pastes, creams, gels, rigid foams, and suppositories fall under this category. Semi-solid dosage forms contain one or more active ingredients, which are uniformly dispersed in a suitable base and excipients, like emulsifiers, viscosity-increasing agents, antioxidants, antimicrobial agents, or stabilizing agents. These semi-solid forms can be available in two forms-
- Conventional semi-solid dosage forms
- Novel semi-solid dosage forms
Semi-solids show a prolonged effect at the application site as they cling to the skin surface until an external force is applied to wash off the drug. Novel semi-solid forms cause less irritation to the skin as they are made up of water washable non-greasy bases. Therefore, they are much superior as compared to conventional semi-solid dosage forms.
Semi-solid dosage forms are viscous dosage forms intended to perform therapeutic, cosmetic, or protective functions. These are dermatological preparations intended for topical and transdermal drug delivery. These are intended for skin, nasal, vaginal, and rectal cavities.
Ointments are oil-based homogenous semi-solid formulations used to induce a therapeutic effect by delivering active ingredients or medications to the targeted or affected site. It is highly suitable for skin diseases and can cure infections. As they have a greasy consistency, they can appear stiff in nature. Apart from therapeutic action, they can also be used as emollients, protective, and prophylactic purposes. Ointments are subcategorized into three main parts, namely, epidemic, endodermic, and diadermic. Epidermic ointments are therapeutic topical ointments that act only on the skin’s surface, unlike endodermic ointments that are also applied topically but can easily penetrate into the skin to act as stimulants or emollients. Diadermic ointments are absorbed thoroughly into the skin to produce systemic effects. Nitroglycerine ointment is an example of diadermic ointments.
Creams are basically opaque viscous emulsions, emulsions, which are applied on skin or mucus membranes, and these can be either w/o (water in oil) type emulsion or o/w (oil in water) type emulsions.
Pastes are highly thick and stiff formulations containing large amounts of finely powdered solids such as starch and zinc oxide. They are highly stiff as compared to ointments.
Gels are jelly-like colloidal semi-solid suspensions meant to be applied on the skin. Gels are clear and homogenous in nature that act as a medicament or as a lubricant.
A poultice is another name for cataplasms. These are soft viscous wet masses of solid substances known for their fomentation action, which helps in reducing inflammation and relieving pain.
Plasters are semi-solid masses that have direct contact with the skin to induce prolonged effects as they are clung to the skin surface. These are basically intended for external application and are attached to the skin surface as dressing.
Suppositories are semi-solid dosage forms that are used through different routes like the rectum, vagina, or urethra. These are not meant for oral use. Vaginal suppositories, also known as pessaries, come in various shapes and sizes. These fit in the vagina and support the vaginal tissues. Urethra suppositories are called Bougies. These are thin, long, cylindrical pencils in shape and are very rarely used. It is mainly used by men facing erection problems. Women’s urethral suppositories are comparatively shorter than men’s. Among these, rectal suppositories are the most common type of suppository as they can deliver many types of medicines.
- As these are topical applications; therefore, the risk of side effects is less.
- They are directly applied to the affected area. Therefore, these have site-specific action.
- It is convenient for patients who are unable to take oral medications.
- Hepatic metabolism is avoided in semi-solid dosage forms due to their topical action.
- They possess higher stability than liquids.
- The highly viscous nature of these forms makes their application easier.
- These can be used to perform therapeutic, cosmetic, or protective functions.
- Less stability than solid dosage forms
- These might cause irritation or allergy in some patients.
- The base used in such forms can be easily oxidized.
- Finger application may cause contamination.
- There is no dose accuracy in such forms.
- They are bulky to handle.
Ideal properties of semi-solid dosage forms
1. Physical properties
- Smooth texture
- Graceful appearance
2. Physiological properties
- The sensitization effect should be low
- Easily miscible
- Should not alter skin functioning
3. Application Properties
- High aqueous washability
- Easily applicable with effective drug release
Ingredients required for semi-solid dosage forms
Bases are the primary and significant ingredient in any semi-solid dosage form. Ointment bases not only act as the carriers of the medicaments but also control the extent of absorption of medicaments commixed in them. Ideal bases contain the following characteristics-
- It must be Inert, non-irritating, and non-sensitizing in nature.
- It should be compatible with skin pH and the drug.
- It must be a good solvent and/or emulsifying agent.
- It should possess emollient, protective, non-greasy, and easily washable functions.
- Release medicament effectively at the site of application.
- It should be pharmaceutically elegant and appealing with higher stability.
According to the USP (United States Pharmacopeia), bases can be classified into four sub-categories, namely, Hydrocarbon bases (oleaginous bases) (Petrolatum, Paraffin, Lanolin, etc.), Absorption bases (cold cream, anhydrous lanolin, etc.), Water-removable bases ( oil in water), and Water-soluble bases (polyethylene glycol).
Preservatives are added mainly to restrict the growth of organisms and to keep the drug preserved from microorganisms for a longer period of time. Para-hydroxybenzoate (parabens), phenols, benzoic acid, and sorbic acid are a few examples of preservatives. Preservatives increase the shelf life of the drug.
A humectant is a hygroscopic substance that is useful for increasing the penetration of drugs through the skin by increasing the solubility levels of active ingredients. These may also help to improve skin hydration.
Oxygen is one of the highly reactive atoms, that has the ability to react and become part of potentially damaged molecules called “Free radicals.” These free radicals can attack the healthy cells of the body and can break their structure, which can affect their functions, for example, Butylated hydroxyanisole, and Butylated hydroxytoluene.
Emulsifiers (emulgents) are substances used to increase the stability of an emulsion by inhibiting flocculation, creaming, and coalescence (breaking, and cracking) in an emulsion. It occurs as a result of increased kinetic stability due to emulgents present. Emulsifiers also help in reducing the interfacial tension between the two surfaces of an emulsion. They are also capable of increasing viscosity at low temperatures. Common emulsifiers used in the preparation of semi-solid dosage forms are Sodium lauryl sulfate (an Oil/Water emulsion), Sodium stearate, and calcium stearate. Glyceryl monostearate is a weak Water/Oil emulsifying agent and is used as a stabilizer and emollient in the Oil/Water emulsion.
6. Gelling agents
Gelling agents are polysaccharides, which have the ability to form gels. We can also call them solidifiers as they provide a semi-solid structure, helping in maintaining the texture of the formulation.
Creams and ointments, when kept under storage for a long time, may undergo pH changes and can reduce the stability of formulations. To maintain stability, buffers, such as citric acid, sodium citrate, or phosphoric acid/sodium phosphate, are added during the process of manufacturing of semi-solid dosage forms.
8. Permeation enhancers
Skin can act as a barrier for the drug to penetrate through the skin. Some of the ingredients like Oleic acid are used as permeation enhancers to enhance the penetration of the drug.