Alexandrite 755 nm

Status: draft compiled 2026-04-20.

The alexandrite laser at 755 nm is the workhorse wavelength for laser hair removal on Fitzpatrick I-III skin. It was among the first clinically successful hair-removal lasers, and despite three decades of competition from diode and Nd:YAG systems, it remains the gold-standard choice for light skin and the single best-studied wavelength for speed and short-term efficacy. It is also the wavelength with the highest paradoxical-hypertrichosis signal on face and neck, the wavelength most implicated in burns on mis-classified Fitzpatrick IV skin, and the wavelength around which most of the litigation history of laser hair removal concentrates.

The device class

Commercial alexandrite platforms include the Candela GentleLASE, GentleMax, and GentleMax Pro (the last adds 1064 Nd:YAG in the same chassis, making it the dominant dual-wavelength platform), the Deka Motus AX (alexandrite-only), and the Cynosure Elite series. Wavelength is fixed at 755 nm by the alexandrite crystal lasing properties; spot sizes typically 10-18 mm; pulse durations 2-20 ms standard with some systems extending to 40 ms; fluences 15-50 J/cm². Cooling is typically Candela's DCD cryogen spray on Candela platforms and sapphire contact on others. Sources: Candela GentleMax Pro product literature; Deka Motus AX product literature. Confidence: C2.

Efficacy

The alexandrite evidence base is among the deepest in cosmetic laser literature because alexandrite has been in clinical use continuously since the late 1990s. Long-term series (Lloyd / Eremia 2001, PMID 11737124) document 73-79% reduction across Fitzpatrick I-III on coarse terminal hair at 6-12 months post-course. Husain 2022 systematic review, PMID 35634805 reports pooled long-term reduction 35-84.2% across studies. The Kao 2023 network meta-analysis, PMID 37493187 (13 RCTs, n=652) found alexandrite performance indistinguishable from diode and Nd:YAG on head-to-head comparisons when matched to appropriate skin type.

The clinical observation from thousands of patient-courses is that alexandrite produces faster visible clearance than diode or Nd:YAG on the same light-skinned coarse-dark-hair patient, but the end-of-course percentage reduction converges. If time-to-visible-result matters (patients managing anxiety about visible clearance), alexandrite is typically the best choice. If end-of-course reduction is the primary outcome, any of the three wavelengths matched to skin type is reasonable. Sources: Lloyd 2001; Husain 2022; Kao 2023. Confidence: C1 for comparable end-of-course efficacy; C2 for faster time-to-visible.

The Fitzpatrick safety envelope

Alexandrite is appropriate for Fitzpatrick I-III and, with caution and reduced fluence, Fitzpatrick IV. It is contraindicated for Fitzpatrick V-VI: epidermal melanin absorbs strongly at 755 nm, the epidermis burns before the beam reaches the follicle, and the resulting complication profile (superficial burns, persistent PIH, blistering, occasional hypopigmentation) is the largest preventable-harm pattern in the history of the modality. Clinics that use alexandrite on patients they have incorrectly classified as Fitz III-IV (typical of clients with tanned skin or Mediterranean, Middle Eastern, South Asian, and mixed ancestry who present as lighter than their melanised baseline) produce burns and scarring that generate most of the litigation in this space. The single most important clinical skill for an alexandrite operator is Fitzpatrick classification discipline: when in doubt, use Nd:YAG. Sources: Dorgham 2020, PMID 31587390; Alster 2006. Confidence: C1.

The paradoxical hypertrichosis signal

Alexandrite is the wavelength with the highest paradoxical-hypertrichosis risk on face and neck. Pooled systematic-review prevalence is about 3% (2021 systematic review, PMID 34057666), but active-screening prospective cohorts in at-risk demographics give dramatically higher rates: Alsarhan et al 2025, Lasers Med Sci, PMID 40405001 found 16.2% in Jordanian women after long-pulse alexandrite, with significant associations to irregular menses, family history of hirsutism, and Fitzpatrick III-IV skin; Moriguchi 2025, PMC12040530 reported 33.3% in male patients (vs 9.0% in women) on back, upper arms, and shoulders with a dual-wavelength Candela GentleMax Pro. Confidence: C2.

The proposed mechanism is sub-therapeutic fluence activating dormant vellus follicles into terminal-hair production, possibly with a hormonally-sensitive component (more common in face/neck regions that are androgen-responsive, and in patients with underlying hyperandrogenism). Full discussion is in ../complications/paradoxical-hypertrichosis.md. Mitigation when using alexandrite in at-risk patients: counsel on the risk explicitly; use higher fluences within the safety envelope (avoid sub-therapeutic dosing); have a plan to switch to Nd:YAG or to electrolysis if paradoxical growth appears; investigate for hyperandrogenism if paradoxical growth is accompanied by other hirsutism features. Confidence: C3.

Pulse duration selection

Standard alexandrite pulse durations for hair removal are 3-20 ms for bulk work and 40-100 ms for longer-pulse modes on some platforms. Shorter pulses (3 ms) on fine hair with thin shafts, where TRT is short and long pulses would over-expose adjacent epidermis. Longer pulses (20-40 ms) on coarse hair and on slightly darker skin types where the extended pulse allows more efficient heat conduction to the bulge while the sustained pulse keeps peak fluence below epidermal injury threshold. See ../physics/thermal-damage-time.md for the full TDT framework. Confidence: C2.

Fluence selection

Typical alexandrite fluences are 15-30 J/cm² for face and small-area work, 20-40 J/cm² for bulk trunk and limbs, and 30-50 J/cm² for thicker coarse-hair bulk (back, chest in some males). The fluence-skin-type relationship is close to linear in the safe range: higher Fitzpatrick phototypes require lower fluences at the same pulse duration, or equivalently longer pulse durations at the same fluence. Matched to appropriate skin type, clinical endpoint is perifollicular edema and mild erythema immediately post-pulse; a stronger endpoint (blistering, urticarial wheals) indicates excessive fluence or inadequate cooling. Sources: Candela fluence guidelines; Dierickx 2000, PMID 10903915. Confidence: C2.

Cooling

Alexandrite platforms are almost all paired with active cooling — either Candela's DCD cryogen spray or sapphire contact cooling built into the handpiece. DCD delivers a 30-100 ms spray of liquid R134a cryogen immediately before each pulse, chilling the epidermis to about 0 °C without freezing. Sapphire contact delivers continuous cold against the skin. Either makes higher fluences safer; the Candela DCD system is the dominant pairing for alexandrite in most US markets. Forced cold air (Zimmer Cryo 6) is used as an adjunct in some practices. Without active cooling, alexandrite at useful fluences burns; the cooling is not optional. See ../protocol/cooling.md. Confidence: C2.

Best-use summary

Alexandrite 755 is the first-line choice for Fitzpatrick I-III patients with coarse, dark, terminal hair on any body region. It is the fastest-clearing wavelength in these populations and has the deepest clinical track record. It is not the right choice for Fitzpatrick V-VI (use Nd:YAG), for blonde/red/grey/white hair (use electrolysis), for any patient with active tanning or recent sun exposure (defer or use Nd:YAG), or for a face/neck treatment course in a patient with hirsutism features or Mediterranean / Middle Eastern / South Asian ancestry where paradoxical hypertrichosis risk is elevated (use Nd:YAG first-line or counsel and monitor). Properly applied, alexandrite is the shortest-course and highest-patient-satisfaction laser hair removal wavelength; mis-matched to skin type or used without adequate cooling, it is the single worst wavelength for preventable-harm potential. Confidence: C1.