Mederma Advanced Scar Gel is said to work by using onion bulb extract as the scar-changing ingredient. It contains water, thickener, alcohol, onion bulb extract, fatty acid, fragrance, and preservative, so it's not the most elegant formula around.
However, believing that it works to make scars look better due to its onion extract may not lead to satisfaction because the research on said onion extract is mixed.
On one hand, a comparative study examining the effects of Mederma’s onion extract with plain Vaseline revealed no difference in scar appearance at the outcome, and other studies have disproven Mederma’s claims about onion extract as well (Sources: Dermatologic Surgery, February 2006, pages 193–197; and Archives of Dermatology, December 1998, pages 1512–1514).
On the other hand, there are studies that show Mederma's onion extract may reduce inflammation and impact collagen remodeling which could reduce the appearance of a scar (Sources: The Journal of Clinical and Aesthetic Dermatology, June 2012, pages 18-23; and Plastic and Reconstructive Surgery, July 2002, pages 177-183). Due to mixed results from the research, we've softened our recommendation on Mederma: Although it's not a slam-dunk, it may prove helpful for some people, but you may have even better results from applying a scar-reducing serum loaded with beneficial skin-healing ingredients, so some experimentation may be needed.
For those curious about the study comparing Mederma treatment with using plain Vaseline, the results were as follows: “Treated [Mederma] and placebo [untreated] subjects were compared on all covariants: age, gender, ethnicity, scar age, and use. No significant difference exists between treated and placebo groups for any of these variables.… More placebo patients than treated patients reported improvement with a less noticeable scar [after] 1 week and a less red scar after 1 month.” Interestingly, “More treated patients reported improvement with a softer scar after 2 months. There were no differences in improvement for either of the physician-related measures between the two groups.”
Additionally, “In this side-by-side, randomized, double-blind, split-scar study, the onion extract gel did not improve scar cosmesis or symptomatology when compared with a petrolatum-based ointment.”
Regarding Mederma’s alleged ability to reduce the unsightly redness some scars present, a detailed study revealed that “Computer analysis of the scar photographs demonstrated no significant reduction in scar erythema [redness] with Mederma treatment.”
Mederma must have seen these studies and taken their disappointing results seriously, because they are now using cosmetic claims, such as “reduce the appearance of scars,” rather than stating directly that their product eliminates or flattens scars to the point where they’re not a visible distraction.
Remember, even after a scar forms, it can and often does improve in appearance over a period of several months and up to two years (the exception to this is, unfortunately, stretch marks and indented, ice-pick scars from acne).
Lastly, the addition of alcohol to the formula (previous versions were alcohol-free) is truly disappointing because alcohol causes free-radical damage and can hurt skin's ability to repair itself (which is critical when the goal is as minor a scar as possible). The amount of alcohol isn't enough to earn this a poor rating, but there are better scar-reducing products to consider first.