The experts I consulted agree that the science behind lip-plumping glosses starts with the ingredients, which cause a mild dermatitis — or skin inflammation — that makes the lips appear larger.
Esthetician and on-air beauty expert Liz Kennedy says, "The ingredients are proven to be efficient in hydration, and [the] different irritants swell the lips." She continues, "Hyaluronic acid brings hydration to the lips, because when the acid is applied topically, it holds 10 times its water weight. The same effect happens to the lips [when you apply a lip plumper that contains hyaluronic acid]."
Clinical dermatologist Dr. Dhaval Bhanusali adds a precautionary note, saying, "The mild contact dermatitis causes the lips to appear larger. The issue is that we never know how much of a reaction a person can have, and [I] see many patients come in with issues including infections from these."