Two things get muddled constantly in laser and IPL hair removal, and the confusion has inflated one of the most over-reported “side effects” in the field. The line is actually simple:
Synchronised growth happens where you treated. Paradoxical hypertrichosis (PH) happens where you didn’t.
That single distinction settles most of the argument. The trouble is that the published literature never draws it cleanly — and the prevalence figures suffer for it.
What synchronised growth actually is
A normal skin field cycles out of step: some follicles in anagen, some in catagen, some resting. That staggering is why regrowth usually trickles in.
Fire repeated sub-lethal fluences into that field and you don’t kill every follicle. Many survive, but take enough of a thermal hit to be knocked into catagen, then telogen. Across several sessions the survivors drift toward the same phase, and when they re-enter anagen they do it together — producing a sudden flush of hair that looks like the treatment has failed or backfired.
It hasn’t. These are the same follicles you always had, arriving on one bus instead of one at a time. There are no more of them. It’s the same illusion that makes people swear waxing thickens regrowth: it doesn’t — it just partially synchronises it.
And this is the part worth shouting out loud about: synchronised regrowth is actually an opportunity. When a large fraction of follicles sit in anagen at once — melanin-rich, matrix intact — they’re at their most vulnerable. Hit them hard at that moment and you clear far more in a single pass than you would on a desynchronised field. Remember, too, that a hair needn’t be visible to be treatable: a late-anagen follicle already carries enough melanin to absorb before the shaft even emerges.
What PH is — and isn’t
True PH is new terminal hair in areas that were never treated — usually adjacent to the treated field, on the face and neck, in darker skin types and patients with hormonal drivers. It’s rare, and its mechanism remains genuinely unresolved.
The bridge between the two is straightforward. At the edge of the beam, scattered sub-lethal light spills into neighbouring untreated follicles. Below the destruction threshold but possibly within a stimulatory band, that low dose may nudge dormant or vellus follicles toward terminal growth. Same root cause as synchronisation — sub-lethal dosing — just landing outside the treated zone instead of inside it.
Where the numbers go wrong
Reported PH prevalence runs from 0.6% to 10%. A tenfold spread isn’t the signature of a well-defined condition; it’s what you get when studies count different things under one name.
The cause is due to ‘definition’. The standard clinical reference (Desai et al., 2010) defines PH as excess hair in treated areas or areas close to them — but “in the treated area” is exactly where synchronised regrowth appears. The definition itself sweeps synchronisation into the PH bucket. Snast et al. (2021) pooled nearly 10,000 patients to a 3% figure, but with heterogeneity so extreme (I² ≈ 97%) that the studies clearly weren’t measuring one consistent thing. By contrast, Inoue et al. (2024), specifically hunting growth above baseline, found genuine cases in just 0.34% of over 7,000 patients — 24 of 25 resulting from use of the alexandrite laser.
My reading: the low figures are real PH. The headline 3–10% figures are real PH plus a large pile of synchronised regrowth that was never PH at all.
The chairside test
When a client/patient returns convinced the laser has made things worse, you can usually settle it in seconds:
| Synchronised Growth | Paradoxical Hypertrichosis | |
| Where | On/in the treated area | Outside of treated areas (usually adjacent) |
| The hairs | Same hairs, re-emerging together | New terminal hairs, not seen before |
| Follicle count | No increase – merely redistributed overtime, but appears as more than before! | A real increase in number of hairs |
| Pattern | A coordinated flush weeks after a session | Progressive new growth over the treatment course |
| Who | Anyone | Darker skin tones with coarser dark hair |
| Device link | Any device! | Usually alexandrite lasers |
| What it means | A real opportunity to kill many follicles simultaneously | A rare but real adverse effect – reassess, consider hormonal referral |
| What to tell your clients | “Not morehair – you are seeing the same hairs growing back together. This is good for a laser/IPL treatment.” | “A genuine increase outside the treatment area(s). Very incommon but manageable. Need to be trated as ‘new’ hair.” |
Keep the PH conversation for the genuine minority in the right-hand column. For everyone else, the honest — and reassuring — answer is that the hair simply learned to march in step!! But that can be a good thing…
Hope this helps,
Mike.
PS We will be talking about this in an upcoming podcast episode…
References:
Desai et al., Dermatol Surg 2010;36(3):291–298 ·
Snast et al., Am J Clin Dermatol 2021;22(5):615–624 ·
Inoue et al., Aesthet Surg J 2024;44(5):NP347–NP353 ·
Moreno-Arias et al., Dermatol Surg 2002;28:1013–1016
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