Laser/Light and "permanent hair reduction"
In 1998, the U.S. Food and Drug Administration (FDA) began allowing some manufacturers of hair removal lasers to use the term “permanent hair reduction."  FDA points out this is different than permanent hair removal:
"Several manufacturers received FDA permission to claim, ‘permanent reduction,’ NOT ‘permanent removal’ for their lasers." 
Richard Felten is currently the FDA regulator responsible for lasers and light-based devices used for epilation. He recently supplied me with FDA’s definition:
"Permanent hair reduction is defined as the long-term, stable reduction in the number of hairs regrowing after a treatment regime. The number of hairs regrowing must be stable over a time greater than the duration of the complete growth cycle of hair follicles, which varies from 4-12 months according to body location. Permanent hair reduction does not necessarily imply the elimination of all hairs in the treatment area." 
Mr. Felten adds that to receive this clearance, FDA requires that hair counts be measured “at 3, 6, 9, and 12 months following the last treatment.” 
The original FDA clearance for "permanent hair reduction" was based on a submission by Palomar Medical Technologies, Inc. , which has a "strategic partnership" with Massachusetts General Hospital (MGH). MGH licenses Palomar the rights to the technology, and in addition to paying the licensing fee, Palomar gives MGH $475,000.00 a year in a contract to do research.  Obviously, it’s in both groups’ financial interests to promote good results, and some have suggested that this type of partnership should be more openly disclosed in published medical articles. 
This original MGH study observed that some subjects had a permanent reduction in the number of terminal hairs, which were observed to have been "miniaturized" into vellus hairs. This type of arbitrary non-standard definition is the reason why it was necessary to define "hair" at the onset of this section. Results depend on what your definition of "hair" is. Here is MGH/Palomar’s:
"We suggest, and hereby use, the following specific definition: "permanent" hair loss is a significant reduction in the number of terminal hairs after a given treatment that is stable for a period longer than the complete growth cycle of hair follicles at the given body site." 
FDA realized most consumers would not make this distinction. Therefore, FDA reviewer Richard Felten notes in his analysis of the data:
"Because of the potential for abuse of the terminology during advertising, it was decided that a statement concerning the indication for use and what was not being granted should be included in the SE letter." 
FDA also required Palomar to put the MGH data into table form, which gives a better picture of the actual results:
Study 1 [6, 8]
Findings: 13 "ideal" subjects (light skin and dark hair)
- 69% were not observed to have permanent hair reduction. Four of these did not complete the study for unspecified reasons.
- 31% were observed to have from 42% to 60% reduction in terminal hairs after two years after 1 treatment.
- Permanent hair reduction was not typical in this study.
Study 2 [unpublished]
Findings: 50 "ideal" subjects
- 54% of those treated at 30 J/cm2 or higher were observed to have 0% to 10% reduction in terminal hairs 12 months after 2 treatments.
- 27% did not respond to two treatments.
- Permanent hair reduction was not typical in this study.
In addition, Study 1 was the basis of two published articles, one of which proclaimed "permanent hair removal" in the title but used their qualified definitions of "permanent" and "hair" in the body of the article.  This article by Dierickx and others was the subject of an editorial appearing in the same journal issue. Written by two experts on alopecia, it took a critical stance regarding the methodology and assumptions made on these limited results:
"Herein lies a very real criticism of the article by Dierickx et al, who conclude that the permanent hair removal they observed resulted from conversion of terminal hairs to nonpigmented velluslike hairs. While they cite an inversion of the terminal-velluslike hair ratio and decreased average hair diameter from pretreatment to 1 year after treatment, these findings are based on 8 hair follicles counted in 2 biopsy specimens taken from a single patient. In our opinion, these are too few hairs upon which to hang one’s hat." 
Tope and Hordinsky conclude: "Unfortunately, appropriately rigorous studies may be difficult to perform in the face of existing FDA market clearance of many hair removal systems."
Some consumers do experience "permanent hair reduction" from laser hair removal. The term can be useful for describing potential laser results if it’s explained properly. However, some consumers do not experience permanent hair reduction, and it’s still unknown why even some "ideal" candidates do not have this response. Unfortunately, some laser promoters misuse the phrase "permanent hair reduction" in ways that are inaccurate and confusing to consumers. This leads to unrealistic expectations in some cases, followed by inevitable disappointment when results don’t match what consumers have been led to expect.
- FDA Docket K980517. 21 July 1998. See summary (requires Adobe Acrobat).
- Laser Facts. June 1999: FDA Center for Devices and Radiological Health.
- Richard Felten, personal correspondence, 17 April 2001.
- PMTI SEC 10-K Annual Report, 26 March 2001: "The Company entered into an amendment to its existing Clinical Trial Agreement with Massachusetts General Hospital, pursuant to which it will fund a minimum of $475,000 per year for research until August 1, 2004."
- e.g. Krimsky S, Rothenberg LS. Financial interest and its disclosure in scientific publications. Journal of the American Medical Association.1998;280:225-226
- Dierickx CC, Grossman MC, Farinelli WA, Anderson RR. Permanent hair removal by normal-mode ruby laser. Archives of Dermatology. 1998 Jul;134(7):837-42.
- FDA Docket K980517. 21 July 1998. See summary (requires Adobe Acrobat). Richard Felten, internal FDA review, 21 July 1998
- Grossman MC, Dierickx C, Farinelli W, Flotte T, Anderson RR. Damage to hair follicles by normal-mode ruby laser pulses. Journal of the American Academy of Dermatology 1996 Dec;35(6):889-94.
- Tope WD, Hordinsky MK. A hair’s breadth closer? Archives of Dermatology. 1998 Jul;134(7):867-9.