From the soothing relief of an ointment to the cosmetic elegance of a face cream, semisolid dosage forms are everywhere in daily life. They bridge the gap between pharmaceuticals and personal care, delivering drugs through the skin while offering comfort and convenience. Unit 5 dives deep into the world of these formulations, exploring their definitions, preparation, and the factors that determine how effectively they work.
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What Are Semisolid Dosage Forms?
Semisolid dosage forms are pharmaceutical preparations with a consistency between solid and liquid. Unlike tablets or syrups, they are meant for topical application on skin, mucous membranes, or rectal and vaginal routes. Their unique texture allows for localized or systemic delivery of drugs, making them vital for dermatological, cosmetic, and even transdermal therapies.
Classification at a Glance
Pharmacists categorize semisolid forms into:
Ointments – greasy preparations offering occlusive and protective action.
Creams – emulsified systems that are cosmetically acceptable.
Pastes – thick, stiff formulations with a higher solid content.
Gels – transparent or translucent systems with a smooth feel.
Each class has distinct uses and properties, tailored to both medical and cosmetic needs.
The Science of Skin Penetration
For a semisolid formulation to be effective, the drug must penetrate through the skin layers. But this process is not as straightforward as it may seem.
Mechanism of Dermal Penetration
Stratum corneum barrier: The outermost layer of skin acts as the biggest obstacle.
Diffusion process: Drugs move from a region of higher concentration (formulation) to lower concentration (skin).
Partitioning: The drug must partition between the semisolid base and the lipids of the skin to cross successfully.
Factors Influencing Penetration
Several factors decide how well a drug passes through the skin:
Drug properties: Molecular size, solubility, and polarity.
Formulation base: Oil-based ointments enhance occlusion, while gels provide rapid absorption.
Skin condition: Damaged or inflamed skin absorbs drugs more easily.
Environmental conditions: Temperature and humidity can alter absorption rates.
Understanding these principles ensures that drugs delivered via ointments or creams are both safe and effective.
Preparation of Semisolid Dosage Forms
Ointments
Prepared by either fusion (melting ingredients and mixing) or incorporation (mixing drug into the base without heat), ointments provide long-lasting adherence on the skin. They are especially useful for dry or scaly conditions.
Pastes
Thicker than ointments due to a higher proportion of solid materials, pastes act as protective barriers. They are less greasy but more resistant to wiping away, making them ideal for conditions like diaper rash or ulcer protection.
Creams
These are oil-in-water (O/W) or water-in-oil (W/O) emulsions. O/W creams are more acceptable for cosmetic use, spreading easily without greasiness, while W/O creams are preferred for moisturizing and therapeutic actions.
Gels
Modern and highly popular, gels use gelling agents like carbopol or cellulose derivatives. They provide a non-greasy, cooling effect and allow rapid drug release, widely seen in acne treatments, pain-relief gels, and cosmetic applications.
Excipients – The Unsung Heroes
Every semisolid formulation requires carefully selected excipients to enhance stability, usability, and effectiveness.
Bases: Petrolatum, paraffin, lanolin, polyethylene glycols.
Emulsifiers: To stabilize creams, such as sorbitan esters and polysorbates.
Humectants: Glycerin or propylene glycol to prevent drying.
Preservatives: Parabens and benzyl alcohol to protect against microbial growth.
Stabilizers and antioxidants: Butylated hydroxyanisole (BHA), tocopherol.
The correct balance of these ingredients ensures that semisolid forms remain stable, appealing, and therapeutic.
Evaluation of Semisolid Dosage Forms
Before reaching pharmacy shelves, semisolid preparations undergo strict quality control to guarantee safety and efficacy.
Physical evaluation: Appearance, color, homogeneity, and phase separation.
Consistency and viscosity: Determines spreadability and patient acceptability.
Drug content uniformity: Ensures each dose delivers the intended therapeutic effect.
Spreadability tests: Measure how easily the preparation can be applied to the skin.
In vitro diffusion studies: Assess how effectively the drug will penetrate the skin.
Stability testing: Monitors resistance to microbial growth, oxidation, and environmental stress.
These evaluations confirm that semisolid dosage forms meet both regulatory standards and patient expectations.