Why does facial laser stimulate hair growth?

It depends on the type of light/energy we’re talking about and the treatment’s objective. I’ll explain it clearly and with evidence:

1) Low-energy lights/lasers (LLLT/”red light”)

•These do not remove hair; on the contrary, they are used to stimulate the follicle (alopecia treatment).

• Proposed mechanism: photobiomodulation leading to increased ATP production, vasodilation, and improved nutrient delivery to the follicle; it promotes re-entry into anagen and prolongs anagen. Clinical trials have demonstrated benefits in both men and women; the FDA has authorized LLLT devices for AGA. (PMC; JCAD; Stanford Medicine)

2) Lasers/light for hair removal (Alexandrite, Diode, Nd:YAG, IPL)

• Their objective is to destroy the follicle through selective photothermolysis.

• Rare exception on the face/neck: “paradoxical hypertrichosis” (growth of new or thicker hair near the treated area).

• Estimated prevalence: meta-analysis ~3% (95% CI 1–6), strongly associated with the face/neck; outside these areas, it is very rare (~0.08%). (PubMed)

• Other reviews report a range of 0.6–10%, with increased risk in skin types III–VI, dark terminal hair, and hormonal conditions (e.g., PCOS). Likely mechanism: subtherapeutic heating and inflammation that trigger the hair cycle rather than destroy it. (Lippincott Journals: PubMed)

• A recent clinical study suggests that the prevalence of facial alexandrite may be higher than historically reported; risk factors include PCOS, menstrual irregularities, darker skin types, and lack of photoprotection. (PubMed; Research Gate)

3) Factors that increase the risk of “growth” after facial laser hair removal

• Low energies/fluences or inadequate parameters (heat without destruction).

• Fine/mixed hair (vellus-terminal) typical of female cheeks/jaw.

• High phototype (III-VI) and/or hyperandrogenism/PCOS.

• Irregular intervals and sun exposure (possible role). (PMC; PubMed)

4) How to minimize it in clinical practice

• Appropriate device and parameter selection (sufficient fluence to reach the follicular destruction threshold).

• Wavelength appropriate to the phototype (e.g., Nd:YAG 1064 nm for high phototypes).

• Session schedule and strict photoprotection.

• Consider hormonal/endocrine management when applicable and supportive topical eflornithine in selected cases. (Clinical review of challenges and solutions in facial hair removal). (PMC)