Propylene glycol is used as a solvent in many pharmaceuticals, including oral, injectable, and topical formulations. Many pharmaceutical drugs which are insoluble in water utilize propylene glycol as a solvent and carrier; benzodiazepine tablets are one example.
Propylene glycol is “generally recognized as safe” by the U.S. Food and Drug Administration (FDA) (FDA 2017). FDA considers an average daily dietary intake of 23 mg/kg of body weight to be safe for persons 2–65 years of age (ATSDR 2008). Various foods, cosmetics, and pharmaceutical products contain propylene glycol.
One more thing: According to the Material Safety Data Sheet on propylene glycol, the chemical is a strong skin irritant, and has been implicated in contact dermatitis. The sheet goes on to warn that the ingredient can inhibit skin cell growth and damage cell membranes, causing rashes, dry skin, and surface damage.
Propylene glycol is tolerated well by the skin and shouldn't cause redness or irritation. Propylene glycol is a penetration-enhancing humectant. This means that it may also help other ingredients to permeate the skin where they would not normally be able to.