Onion extract: in both pieces of material we found, this ingredient is not standing on its own

A comparison collated by the 2021 Cochrane review, which set silicone sheeting against ten things, one of them topical onion extract.

One trial, 32 participants, entered this comparison.

Result: silicone gel sheeting may slightly reduce severity of scarring compared with topical onion extract — mean difference −1.30 (95% CI −2.58 to −0.02), certainty rated low by Cochrane.

How to read it.

First, this is the only head-to-head we found. And onion is the side that lost. Since onion extract and silicone sit next to each other on the shelf, that is practical information.

Second, the margin is small. One end of the interval is −0.02. All but touching zero. There is no room to say silicone is clearly better.

Third, 32 people at low certainty. Cochrane downgraded once for risk of bias and once for imprecision.

Fourth, and this matters — there is no 'nothing' arm in this comparison. Onion having done less than silicone, and onion doing more than nothing, are different questions. We did not find an answer to the second.

So it sits at 'not established'. What this entry says goes as far as 'set against silicone, onion comes in slightly behind'.

Read the silicone page alongside. That is a page where Cochrane rated all of the evidence low or very low. The winner's evidence is not thick either.

A 2022 trial in Thailand. Prospective, randomised, double-blind, over three months.

The design is unusual. Fully closed post-caesarean wounds were divided into two halves along the closure axis, and each half was randomly assigned.

• one half — silicone gel containing 5% onion extract
• the other — silicone gel containing vitamin C

Assessed at one, two and three months.

Results:

• after three months of follow-up there was a statistically significant improvement in the scar between before and after treatment
• but between the two sides, no significant difference was found in POSAS (Patient and Observer Scar Assessment Scale), VSS (Vancouver Scar Scale) or melanin value
• no adverse effects were reported in either group

How to read it.

First, and this matters most — both sides were silicone gel. The trial compared silicone gel with onion in it against silicone gel with vitamin C in it. So the question it can answer is 'what should be added to silicone' — and 'does onion work on scars' is outside this design.

Second, do not read 'improved from before to after' as a result. Both sides improved. But a caesarean scar changes considerably on its own over three months. And silicone was covering both sides. The improvement in a before-and-after comparison cannot be moved into onion's column.

Third, the abstract does not give a participant count. This site does not invent numbers it does not have, which is why the participant field on the badge is empty.

Fourth, the endpoints mix two kinds. POSAS is scored by patient and observer together, VSS by an assessor, and melanin by an instrument. None of the three differed.

So it sits at 'not established'.

The split result of the same trial.

In the entry above, POSAS, VSS and melanin did not differ. But one thing did.

Improvement in scar erythema was significantly greater on the side with the silicone gel containing 5% onion extract.

Why we grade this differently from the entry above.

Because something did split. One of several endpoints reached significance, and it went onion's way. And that one is a measure tied to inflammation. Since what leads the pitch for onion extract is anti-inflammatory, the result does not run against that account.

But we will not read more into it than that. Here is why.

First, it is one of several endpoints. This trial measured POSAS, VSS, melanin value and erythema together. The more endpoints, the higher the chance one reaches significance by luck. We wrote the same on the resveratrol page.

Second, the abstract does not say how erythema was measured. Whether by instrument or as a subscale of a scoring system, we cannot tell. So the endpoint field on the badge reads 'score given by a masked assessor' — and even that is not confirmed.

Third, both sides are still silicone. What this result says is 'adding onion to silicone left less redness than adding vitamin C to silicone'.

Fourth, we do not know how many people there were.

So it sits at 'early evidence'. That something split is worth recording; more than that cannot be.

One last note to close the page. If you are using a scar product containing onion extract, look at the ingredient list to see whether silicone is in it too. In the trial above, a fair share of the improvement may belong to the silicone that was on both sides.