The evidence, side by side
Before any of this: the American Academy of Dermatology is clear that effective treatment for hair loss starts with finding the cause, and that getting an accurate diagnosis means seeing a board-certified dermatologist (AAD, hair loss diagnosis and treatment). Thyroid disease, low iron, hormonal change and medication all cause shedding, and no cosmetic product on this page addresses any of them.
Rosemary oil has one randomized comparative trial. Published in SKINmed in 2015, it randomized 100 participants with androgenetic alopecia to rosemary essential oil or 2% minoxidil. Both groups showed a significant increase in hair count at six months compared with baseline, with no significant difference between the groups, and scalp itching was more frequent in the minoxidil group (Panahi et al., SKINmed 2015).
Peptide serums generally rest on consumer perception studies and ingredient rationale. Vegamour's headline figures come from a 120-day study of 40 participants reporting on their own hair. The Ordinary frames its claim as improving the appearance of thickness and fullness, which is a cosmetic appearance claim rather than a growth claim.
On evidence alone, rosemary is ahead. But "one small trial" is a low bar to clear, not a strong position, and it is cited across the internet far beyond what it supports — our full write-up explains the caveats.
The practical differences
Rosemary oil is an oil. It needs to go on before a wash and be washed out, or it sits on your scalp and attracts buildup. That makes it a once- or twice-weekly ritual rather than a daily habit, and if your scalp is already oily it is the wrong product entirely.
A peptide serum is a leave-in. It absorbs without greasy residue and you apply it daily and forget it. For most people that is a routine they can actually sustain, and sustaining it for six months is the whole game.
There is a second practical point in rosemary's favor: it is genuinely good on a dry scalp and split ends regardless of the growth question. If the growth angle does nothing, you still own a useful oil. A serum that does nothing is just an expensive bottle.
Cost and commitment
We do not quote prices anywhere on this site, because we have no verified live price data and a number we cannot stand behind is worse than none. The stable shape: the rosemary oil is the cheapest thing in this category by a distance, The Ordinary's serum is inexpensive for a peptide product, and the well-known plant-based serums cost several times either. Check all three yourself.
Either way, budget for six months. Both need daily or near-daily use for that long before the result means anything, and consistency matters more than which you chose.
Which we would start with
Rosemary oil, if your scalp is dry or normal and you can live with a pre-wash ritual. It is the cheapest way to find out, it has the better evidence of the two, and it earns its place as a scalp oil even if nothing else happens.
A peptide serum, if your scalp is oily, or if you know from experience that you will not keep up a pre-wash routine for six months. A serum you actually use beats an oil you stop using in week three.
Neither, if the thing you have actually noticed is short broken hairs around your part and a thinner ponytail — that is breakage, and a leave-in will do more for it than either of these.
We have not used these products ourselves. They are chosen from what is in the formula, what those ingredients are documented to do, and whether that matches the problem this page is about — and every product below links to the manufacturer's own page so you can check the formulation claims yourself. The full method is here.