semisolid summary

INTRODUCTION

Semisolid dosage forms include ointments, creams, and gels. They are applied to the skin, eyes, nose, vagina, or rectum.

  • Why they are used:

    • Medicated: deliver drugs for therapeutic effect.

    • Unmedicated: act as protectants, emollients (skin softeners), or lubricants.

  • Effects:

    • Local action → treating skin/mucosal conditions.

    • Systemic absorption → sometimes unintended (important in pregnancy and breastfeeding).

👉 Important distinction:

  • Topical dermatological product → drug acts within the skin (skin is the target).

  • Transdermal product → drug penetrates through skin into systemic circulation (percutaneous absorption). Here, the skin is not the target organ.

OINTMENTS

Definition: Semisolid preps for external use on skin/mucous membranes.

They can be medicated (drug incorporated) or unmedicated (just base, used for protection/emollience).

Ointment Bases (USP → 4 classes):

  1. Oleaginous (hydrocarbon) bases

    • Greasy, occlusive (lock in moisture), long-lasting.

    • Poorly washable with water.

    • Example: Petrolatum (yellow petroleum jelly). White petrolatum = decolorized version.

    • Yellow ointment USP formula: Yellow wax 50 g + Petrolatum 950 g.

    • Term unctuous = oily/greasy texture.

    • Issue: difficult to add water-based ingredients.

  2. Absorption bases

    • Designed to take in water and form W/O emulsions.

    • Either:
      a) Can incorporate aqueous solutions to form W/O emulsions (ex: Hydrophilic petrolatum).
      b) Already W/O emulsions but can take more water (ex: Lanolin).

    • Less greasy than hydrocarbons but still not washable.

    • Example: Hydrophilic petrolatum USP: Cholesterol 30 g, Stearyl alcohol 30 g, White wax 80 g, White petrolatum 860 g.

    • Commercial: Aquaphor, Aquabase.

  3. Water-removable bases

    • O/W emulsions → look like creams.

    • Easier to wash off, can absorb discharge.

    • Example: Hydrophilic ointment USP (contains parabens, SLS as emulsifier, propylene glycol, stearyl alcohol, petrolatum, water).

    • Require preservatives.

  4. Water-soluble bases

    • Greaseless, completely water washable.

    • No petrolatum/greasy material at all.

    • Best for incorporating solid drugs.

    • Example: PEG ointment NF = PEG 3350 (400 g) + PEG 400 (600 g).

    • PEG properties:

      • PEG < 600 = liquid.

      • PEG > 1000 = solid.

      • Intermediate = semisolid.

Choosing a Base → depends on:

  • How fast drug should release.

  • If systemic absorption (percutaneous) is desired.

  • Whether occlusion (moisture-lock) is needed.

  • If drug is stable in base.

  • Base–drug compatibility.

  • How easily base washes off.

  • Application site (dry skin = ointment; oozing = cream; skin folds = lotion).

Preparation of Ointments

Two major methods:

  1. Incorporation → mix drug into base until uniform.

    • Tools: mortar & pestle, spatula + slab, Unguator (electronic), ointment mill.

    • Incorporating solids:

      • Geometric dilution → progressively mix drug with base.

      • Levigation → triturate solid in a little levigating agent (mineral oil or glycerin) to reduce particle size.

      • Pulverization by intervention → dissolve gummy solid (ex: camphor, iodine) in volatile solvent, spread, evaporate, then triturate into base.

    • Incorporating liquids:

      • Only if base can absorb.

      • If base cannot → mix liquid into hydrophilic base first, then into oleaginous base.

  2. Fusion → melt components, cool with stirring until congealed.

    • Add heat-labile/volatile substances last.

    • Emulsion ointments: heat oil phase + water phase to same temp (~70–75°C), then combine with stirring.

    • Large-scale → steam-jacketed kettles.

USP Requirements for Ointments

  • Microbial content: Must be free from pathogens (S. aureus, P. aeruginosa). Ophthalmic ointments must be sterile.

  • Minimum fill: actual contents match label.

  • Packaging/storage: jars/tubes, closed, cool, light-resistant.

  • Labeling: conditions + directions.

  • Tests: viscosity, in vitro release via Franz diffusion cell (membrane + receptor fluid sampling).

CREAMS

  • Semisolid emulsions (W/O or O/W).

  • Vanishing creams → O/W with high water & stearic acid, leaves thin film when water evaporates.

  • More elegant than ointments → spreadable, washable.

  • Uses: skin, rectal, vaginal.

Preparation:

  • Divide into oil and water phases.

  • Heat both, then combine at same temp, stir while cooling.

  • Sometimes homogenized for uniformity.

  • API can be added to appropriate phase or after via levigation.

  • Preservatives required.

GELS

  • Semisolids made of drug in an aqueous vehicle thickened with gelling agents.

  • Gelling agents: Carbomers (carbomer 940 most viscous), cellulose derivatives (CMC, HPMC), gums (tragacanth).

  • Carbomers: crosslinked acrylic acid polymers, 0.5–2% needed for thick gels.

  • Types:

    • Single-phase → uniform distribution.

    • Two-phase (magmas) → contain floccules (e.g., milk of magnesia).

  • Property: thixotropy = thick on standing, thin when shaken.

  • Formulation: may include alcohol/propylene glycol (solvent), parabens (preservative), EDTA (stabilizer).

  • Routes: topical, ophthalmic, nasal, vaginal, rectal.

  • Example: Androgel (testosterone).

TRANSDERMAL PREPARATIONS

  • Deliver drugs through skin into systemic circulation (percutaneous absorption).

  • Challenge: stratum corneum is strong barrier.

  • Solution: penetration enhancers like DMSO, ethanol, propylene glycol, PEG, urea, lecithin, surfactants, terpenes.

  • Example: Pluronic lecithin organogel (PLO) = Poloxamer F127 gel (20–30%) mixed ~1:5 with equal parts isopropyl palmitate + lecithin.

OTHER SEMISOLID DOSAGE FORMS

Pastes

  • Thick semisolid with ~25% solids.

  • Very stiff, absorb secretions, protective.

  • Example: Lassar’s Zinc Paste (25% zinc oxide + 25% starch + petrolatum).

Plasters

  • Adhesive semisolid masses spread on backing.

  • Unmedicated → protection/support.

  • Medicated → local drug delivery (ex: Salicylic acid plaster for corns).

Glycerogelatins

  • Plastic masses: gelatin 15%, glycerin 40%, water 35%, drug 10%.

  • Applied warm, hardens into protective layer.

  • Example: Zinc gelatin boot for varicose ulcers.

PACKAGING & STORAGE

  • Jars (glass/plastic).

  • Tubes (aluminum/plastic). Sizes 1.5–120 g.

  • Ophthalmic tubes: 3.5 g with narrow tips.

  • Syringes: accurate dosing, exclude air.

Filling:

  • Jars → spatula or pouring.

  • Tubes → filled from open end, then crimped/sealed.

  • Syringes → back-filled, plunger reinserted.

CLINICAL USES

Dermatologic

  • Skin = barrier (stratum corneum).

  • Routes of absorption: mainly passive diffusion, minor through hair follicles/glands.

  • Factors: drug concentration, solubility, partition coefficient, base used.

  • Base choice: ointments (dry skin), creams (oozing skin), lotions (friction areas).

  • Patient tips: clean area, thin layer, avoid occlusion unless told, wash hands after, stop if irritation occurs.

Ophthalmic

  • Absorption mainly via cornea, also conjunctiva/sclera.

  • Advantage: ointments/gels ↑ contact time vs solutions.

  • Base: white petrolatum + mineral oil, sometimes lanolin or PEG.

  • Must be sterile and pass metal particle test: ≤50 particles >50 µm in 10 tubes; ≤8 in any one.

  • Preservatives: methylparaben, propylparaben, chlorobutanol, benzalkonium chloride.

  • Application: wash hands, tilt head, pull lid, apply 0.25–0.5 in ribbon, close eye, expect blurred vision.

Nasal

  • Local (decongestants) or systemic (butorphanol, nicotine, cyanocobalamin).

  • Example: Nascobal Gel (B12).

  • High vascularization aids absorption.

Rectal

  • Used for hemorrhoids, pruritus, pain, inflammation.

  • Drugs: zinc oxide, cocoa butter, lanolin, pramoxine, hydrocortisone.

  • May come as ointments, creams, gels, foams.

KEY TERMS