Review of Changes in Topical Drug Product Classification - Pharmaceutical Technology

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Review of Changes in Topical Drug Product Classification
This article summarizes the classification systems for topical liquid and semisolid dosage forms used for dermatological application and notes some differences between FDA and USP classification.


Pharmaceutical Technology
Volume 32, Issue 10, pp. 66-74

Is the semisolid a paste, gel, ointment, or cream?

A paste is defined as "a semisolid dosage form, containing a large proportion (20–50%) of solids finely dispersed in a fatty vehicle. This dosage form is generally for external application to the skin or mucous membranes." For the decision tree, one must as the question, Does the semisolid contain >50% water and volatiles? If the answer is no, then one should ask, Does the semisolid contain a large proportion (20–50%) of dispersed solids? If the answer is yes, then the dosage form is classified as a paste. This "new" definition of a paste is much more detailed than the pre-2006 definition, which states that a paste is a semisolid dosage form that contains one or more drug substances intended for topical application. "Water and volatiles" percentage for all of the semisolid dosage forms is determined by loss on drying when heated at 105 C until constant weight is achieved. Dispersed solids is not defined, but is presumably estimated from knowledge of the components and composition of the formulation.

A gel is defined as "a semisolid dosage form that contains a gelling agent to provide stiffness to a solution or a colloidal dispersion. A gel may contain suspended particles." This definition is footnoted with "A colloidal dispersion is a system in which particles of colloidal dimension (typically between 1 nm and 1 μm) are distributed uniformly throughout a liquid." For the decision tree, one must first ask the question, Does the semisolid contain >50% water and volatiles? If the answer is yes, then one must ask, Is the semisolid a solution or colloidal dispersion stiffened with a gelling agent? If the answer is yes, then the dosage form is classified as a gel.

Ointment is defined as "a semisolid dosage form, usually containing <20% water and volatiles and >50% hydrocarbons, waxes, or polyols as the vehicle. This dosage form is generally for external application to the skin or mucous membranes." For the decision tree, one should first ask, Does the semisolid contain >50% water and volatiles? If the answer is no, then one should ask, Does the semisolid contain a large proportion (20–50%) of dispersed solids? If the answer is no, then one should ask, Does the semisolid contain >50% of hydrocarbons, waxes, or polyols such as polyethylene glycol and <20% water and volatiles? If the answer is yes to both parts of the question, then the dosage form is classified as an ointment.

With this classification system, cream becomes the default dosage form. If one or both of the two criteria for ointment are not meet, then the product is a categorized as acream. Likewise if the criteria "containing >50% water and volatiles required for a gel" is met, but the semisolid is an emulsion, then the product is a cream. Cream is defined as "an emulsion, semisolid dosage form, usually containing >20% water and volatiles and/or <50% hydrocarbons, waxes, or polyols as the vehicle. This dosage form is generally for external application to the skin or mucous membranes."


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