An email from a reader prompted me to look into onion extract and whether it can really fade scars and stretch marks, as well as be generally therapeutic for the skin. He has been using a scar-fading gel called Mederma, which has onion extract and not much else. Amusingly he also mentioned that Ninon de L'Enclos, the seventeenth-century French courtesan and wit, was supposedly still breaking hearts at ninety, thanks to a balm of lard, beeswax and onion juice.  Let’s hope she wiped it off before getting up close and personal.

But aside from a personal anecdote and an appealing myth, is there any hard evidence that onion extract will fade scars and stretch marks? The short answer is no. And what's more, you'd be better off using petroleum jelly than Mederma.

In 2001, a study showed that onion extract used on rabbit ear scars improved collagen organization but didn't lead to a difference in scar appearance.  Other studies have shown that onion extract is no better at healing than petroleum jelly. And one study even used Mederma itself in a side-by-side, randomized, double-blinded, split-scar study. It concluded that “the onion extract gel did not improve scar cosmesis or symptomatology when compared with a petrolatum-based ointment.”

The best redemption of the onion that I could find was research that had success – albeit limited – with onion extract on hypertrophic and keloid scars (keloids are the result of an overgrowth of dense fibrous tissue that usually develops after healing of a skin injury). It helped fade the color, but not the height of the scar.

Now I can see why some scar treatment makers, such as Invicible, go out of their way to say they don’t use onion extract.

Ingredients in Mederma Gel

Water (Purified), PEG-4, Allium Cepa (Onion) Bulb Extract, Xanthan Gum, Allantoin, Fragrance, Methylparaben, Sorbic Acid