Lupine Hammack

ALLANTOIN

The headliner in this gel, which claims to “reduce the appearance” of scars, is allantoin, a nitrogen-rich waste molecule excreted in mammalian urine. It softens keratin, the fibrous protein that makes your birthday suit tough and waterproof. That smooths the skin and encourages dead skin cells to slough off.

ALLIUM CEPA BULB EXTRACT

Researchers think allantoin's main role is to increase the penetration of Allium cepa, aka onion. While not listed as an active ingredient, onion is a longtime additive of choice for scar treatments. You recognize onion by its smelly sulfur-containing molecules, but it's the flavonoids quercetin and kaempferol that might improve your scar—possibly by regulating fibroblasts, the cells that help build scaffolding for new tissue.

PANTHENOL

Onion extract already includes pantothenic acid, or vitamin B5, but Mederma adds panthenol, its functionally identical alcohol, for extra oomph. The namesake of Pantene hair products, panthenol's three-OH groups form hydrogen bonds with water, pulling moisture into skin (or hair). That makes scar tissue more elastic—an essential element of healing—and promotes fibroblast growth, helping new tissue form.

SODIUM HYALURONATE

Another water trapper. The acid version of this salt, hyaluronic acid, shows up in the fluid between skin cells, the eye's gooey vitreous humor, and the lubrication in joints. Your skin's natural levels of the stuff drop over time, which may explain why toddlers' skin can rebound from a scrape so much more quickly than a 55-year-old's.

LECITHIN

Yes, yet another moisture-locking ingredient. Turns out hydration may be more important than an onion bath: As a wound heals, the skin can lose water, sending good fibroblasts into overdrive and creating a raised scar. In addition to drawing water into the skin, lecithin works to prevent water evaporation by forming a fatty barrier over the skin's surface.

METHYLPARABEN AND SORBIC ACID

Keeping tubes of gel meant for sensitive skin free of bugs and mold is important. That's where these two preservatives come in. Methylparaben gets a bad rap for overblown cancer risks, but it's true that medium-wave UV light converts it into radicals and ketenes, highly reactive molecules that could damage the outermost layer of skin. If you're using Mederma, make like a vampire and avoid the sun.