Red light therapy
How long should a red light session be?
Manuals run from a few minutes a day to twenty minutes three times a week. The trials explain some of that spread. They do not crown a perfect number of minutes.
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Follow the device instructions, keep the same schedule, and judge pattern hair loss at about 16 to 26 weeks. Trials often used 15 to 20 minutes, three times a week, for about six months. One meta-analysis found a larger difference versus a sham device in its lower-frequency subgroup than in its higher-frequency subgroup. There is no settled ideal dose, and a longer session is not automatically a better one.
See a GP or doctor if hair loss is sudden, patchy, or comes with illness, a rash, or other symptoms. The NHS asks for that conversation before a private clinic.
On this page
On this page
Use the session length in the device instructions, repeat it often enough to be boring, and judge pattern hair loss after about 16 to 26 weeks. A longer session is not automatically a better session. The trials cluster around 15 to 20 minutes, three times a week, for months, and one pooled analysis found a larger difference against a sham device when the weekly schedule was the lower-frequency one. Nobody has settled a single ideal dose.
This page is about the light and the clock. It does not recommend a brand.
Why do the manuals disagree?
Because the studies disagreed with each other, and because minutes are only half of a dose.
Pillai and Surendran, writing in the Journal of Cutaneous and Aesthetic Surgery, describe the devices most often used for this purpose as wavelengths from 650 to 1200 nanometres, fluences of 1 to 10 joules per square centimetre, and power densities of 3 to 90 milliwatts per square centimetre. They say treatment is usually 15 to 20 minutes, three times a week, for six months.
That “usually” is a description of the literature, not a personal prescription. A comb, a helmet and a clinic panel do not deliver the same power to the same patch of scalp. Ten minutes under a brighter device can deposit a similar energy to a longer sit under a dimmer one. Irradiance, the power on the skin, times time, is the dose. A minute count copied from a different machine skips that sum.
The same review describes a biphasic response, drawn from in vitro studies and animal experiments and from the Arndt–Schulz idea: a small exposure can stimulate, and too much can inhibit. Insufficient irradiance, or too short a time, may do nothing. Too high an irradiance, or too long a time, may have the opposite of the effect people want. Only some balance of power and time is stimulating. That is a concept from laboratory and animal work. It is not a chart that tells you your Tuesday session was 90 seconds too long.
What did a sham-controlled comb trial actually do?
Leavitt and colleagues ran a 26-week, multicentre, sham-controlled trial in 110 men with androgenetic alopecia. The device was used for 15 minutes, three times a week. Terminal hair density rose more with the working comb than with the sham comb. Subjective scores at 26 weeks also favoured the working device.
Fifteen minutes, three times a week, is 45 minutes a week. It sits inside the “usual” band in the Pillai review. It is one protocol, in men, with one comb design. It is not a reason to double the time because a forum said more light must mean more hair.
Did less time per week look better in a pooled analysis?
Liu, Liu, Chen and Lin, in Lasers in Medical Science in 2019, pooled 8 studies that contained 11 double-blind randomised trials of low-level laser therapy for adult androgenetic alopecia. Hair density rose more with the working devices than with sham devices. The standardised mean difference was 1.316 (95% confidence interval 0.993 to 1.639).
The subgroup that matters for this question is frequency. The low-frequency subgroup had a larger difference versus sham (standardised mean difference 1.555, 95% confidence interval 1.132 to 1.978) than the high-frequency subgroup (0.949, 95% confidence interval 0.644 to 1.253). The authors say low treatment frequency had a better hair-growth effect than high treatment frequency. They also say the review was limited by heterogeneity: the trials were not the same experiment repeated.
Liu’s abstract says “low-frequency” and “high-frequency”. It does not, in the abstract, define those words as a number of minutes. A later paper in Actas Dermo-Sifiliográficas, summarising Liu, describes the low-frequency regimen as less than 60 minutes a week and the high-frequency regimen as more than 60 minutes a week. Treat that minute split as the later review’s description of Liu’s subgroups, not as a line you can point to in the 2019 abstract.
Even on that reading, both subgroups still beat sham. The finding is not “stop at 59 minutes or the hair falls out”. It is “in this mixed set of trials, the lower-frequency schedules had the larger average difference”. Device type (comb or helmet) and a short versus long course did not, in Liu’s analysis, change the result in the same way. Wavelengths across the included studies ran from about 630 to 808 nanometres, according to the Actas summary.
When is it fair to judge the result?
Perez and colleagues pooled pattern-hair-loss studies and found higher hair density than placebo. The difference was larger in studies longer than 20 weeks than in shorter ones, and the heterogeneity was high. The practical window used on this site is about 16 to 26 weeks of consistent use. That matches the comb trial’s 26 weeks and the six-month courses the reviews describe. The full numbers are on does red light work.
A good week, or a shed in week two, is not the endpoint. Neither is a heroic first fortnight of extra sessions to “catch up”. Consistency is the part the trials share. The minute count is the part they do not share.
What a minute count leaves out
People ask for one number because a number feels like a prescription. The papers do not offer one that survives a change of device.
Power density in the Pillai review runs from 3 to 90 milliwatts per square centimetre. That is a thirty-fold span. A 15-minute session at the low end of that span is not the same dose as 15 minutes at the high end. Fluence, the energy delivered per square centimetre, is quoted in that review as commonly 1 to 10 joules. Joules are what you get when you multiply power by time. Two devices with the same timer and different diodes are not interchangeable, even if both are described as red.
Coverage is the other missing term. A comb treats a stripe and has to be moved. A cap treats the area under the shell. A clinic panel treats whatever the chair points at the lamp. The Leavitt protocol of 15 minutes assumed the comb was used as that trial used it, parting the hair so the beams reached the scalp. Sitting still under a different lamp for 15 minutes is a different exposure, even when the clock matches.
Liu’s frequency finding, however the later review defines the minute cut-off, was a subgroup of unlike trials. It is a reason to be suspicious of “more is better”. It is not a reason to set a kitchen timer to 59 minutes and call that optimal. The authors’ own limit, heterogeneity, means the tidy cut can move if the next trials are different.
There is also no good evidence on this page for a loading phase, a double session on day one, or a rule that you must never miss a day. The shared instruction in the better studies is repeated use across months. Missed days happen. The useful response is to return to the manual’s schedule, not to compensate with a longer sitting.
What should you do with a device at home?
Read the manual for that device and follow it. If it says ten minutes a day, do ten. If it says fifteen minutes three times a week, do that. Do not add a second session the same day because the first one felt pleasant. Do not mix two devices on the same scalp on a guess.
If the manual and a trial protocol disagree, the manual is the safety instruction for the hardware you own. The trial protocol belonged to a different machine. Questions about wavelength, the stated output in milliwatts per square centimetre, coverage and warranty belong on how to choose a device. The hats roundup and the laser caps and combs page are where specific hardware is compared. Clinic chairs versus a box at home are on clinic or home.
Red light is not an NHS treatment for hair loss. It has been studied mainly in pattern loss. A sudden shed, patches, or illness still belongs with a GP before a gadget.
Common questions
Is a longer red light session better for hair?
Not automatically. Too little light may do nothing, and laboratory and animal work on low-level light describes a point where more exposure becomes inhibitory. Human trials have not settled an ideal minute count. Follow the manual for the device you have.
What schedule did the hair trials use?
A review in the Journal of Cutaneous and Aesthetic Surgery says treatment is usually 15 to 20 minutes, three times a week, for six months. One sham-controlled comb trial used 15 minutes, three times a week, for 26 weeks. Other devices in the wider literature used different times.
What is the under-60-minutes finding?
Liu and colleagues found a larger increase in hair density versus sham in their low-frequency subgroup than in their high-frequency subgroup. A later review describes that split as under 60 minutes a week versus over 60. Liu’s own abstract does not print the minute cut-off, and the authors warned that the trials were heterogeneous.
When should I decide if it is working?
At about 16 to 26 weeks of consistent use, which is when the stronger pattern-hair-loss studies counted hairs. A change after one week is not what those studies were built to show.
Sources
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