About Propylene Glycol
Propylene glycol (PG) is a clear, odorless, and viscous liquid that is miscible with water and many organic solvents. It is a small molecule that is structurally different and unrelated to PEG, despite a similar name. PG is commonly used as an excipient in a variety of drugs and is also approved for use in food and cosmetics. In addition, it has a wide range of other practical applications, such as antifreeze agent, cutting fluid, and as an additive to latex paints and coatings to increase freeze-thaw capabilities.
Propylene Glycol as An Excipient
Excipients are essential supports for active ingredients in drugs, vaccines and other products, contributing to their stability, preservation, pharmacokinetics, bioavailability, appearance and acceptability. Propylene glycol, a derivative of petroleum, is commonly used as an excipient due to its solvency, stability, and low toxicity. Here are some key uses and properties of propylene glycol as an excipient:
Propylene glycol is used as a solvent in various parenteral (injectable) formulations, ointments, creams, and other topical products. It helps dissolve active ingredients and ensures their uniform distribution throughout the formulation.
Propylene glycol acts as a stabilizer in oral and topical suspensions, helping to prevent separation and settling of solid particles. It also helps maintain the physical stability of emulsions, preventing phase separation.
Propylene glycol has hygroscopic properties, meaning it can attract and retain moisture from the surrounding environment. It is often used as a humectant in cosmetics and skincare products, helping to keep the skin hydrated and supple.
- Viscosity Modifier
Propylene glycol can be used as a viscosity modifier to adjust the consistency and flow properties of formulations. By adding propylene glycol, the viscosity of a product can be increased or decreased as desired.
- Preservative Enhancer
Propylene glycol can enhance the effectiveness of certain preservatives in preventing microbial growth in formulations. It can improve the antimicrobial activity and stability of preservative systems.
Propylene glycol can solubilize poorly soluble drugs or ingredients in formulations, improving their bioavailability and efficacy. Propylene glycol is particularly useful in enhancing the solubility of lipophilic (fat-soluble) compounds.
Propylene Glycol: Potential Cause of Allergic Reactions
Although propylene glycol is generally considered a safe pharmaceutical excipient, it may be a potential cause of allergic reactions. Some studies have reported the clinical manifestations of propylene glycol in delayed-type hypersensitivity reactions (DHR). For example, PG excipients in several drug types, including topical corticosteroids, oral antihistamines, and topical steroid creams, may cause allergic contact dermatitis (ACD), as confirmed by patch testing (PT). 
Delayed hypersensitivity reactions to PG excipient. 
- Maria Luisa Caballero, et al. J Allergy Clin Immunol Pract. 2021, 9(8): 2968-2982.
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