Mederma Skin Care for Scars ($35 for 1.76 ounces) is an overadvertised, overhyped cosmetic preparation claiming to improve the appearance of scarring. Onion extract is supposed to be the scar-changing ingredient in a formula that is as mundane and ordinary as you can get; it includes water, thickeners, onion extract, fragrance, and preservative. There is no research showing onion extract to be effective as a skin-care ingredient.
An article in the Archives of Dermatology (December 1998, pages 1512-1514, “Snake oil for the 21st century”) from the Department of Dermatology, Harvard Medical School, stated that “With the current promulgation of skin ‘products’ and their promotion and even sale by dermatologists, and the use of treatments of no proven efficacy, this association between dermatology and quackery is set to continue well into the 21st century. The list of offending treatments includes silicone gel sheets and onion extract cream (Mederma) for keloids ”
Another study (Source: Cosmetic Dermatology, March 1999, pages 19-26) concluded that there were no discernable differences between skin treated with Mederma and skin that was treated with a placebo. Nevertheless, Mederma advertises itself as promising to get rid of scars.
More recently, a study in Dermatologic Surgery (February 2006, pages 193-197) concluded in a “side-by-side, randomized, double-blinded, split-scar study, the onion extract gel [Mederma] did not improve scar cosmesis or symptomatology when compared with a petrolatum-based ointment.”
A customer service representative for Mederma told me that the onion extract “prevents the release of histamines which causes scarring.” Even if onions could prevent the release of histamines, in reality histamines have everything to do with allergic reactions, and nothing whatsoever to do with what causes scarring. The body produces histamines in response to an allergic reaction; the body sends them in to fight the allergen that causes the redness, swelling, and itching. While histamines can cause the skin to react, that does not inhibit scar reaction. If anything, because onions release a complex mixture of sulfur-containing oils together with sulfur-free aldehydes and ammonia, all of which are more or less intensely volatile (that’s what makes your eyes burn and tear when you cut into them), onion extract may be a skin irritant.