Exposure: Roodlichttherapie

Oral mucositis is one of the more debilitating side effects of cancer treatment, and photobiomodulation applied to the oral tissues before or during treatment reduces how severe it gets and how much it hurts. The evidence was strong enough that the Multinational Association of Supportive Care in Cancer and the International Society of Oral Oncology built it into their mucositis guidelines: recommended to prevent oral mucositis in adults undergoing hematopoietic stem-cell transplantation with high-dose conditioning chemotherapy, in patients having radiotherapy without chemotherapy for head and neck cancer, and in patients having radiotherapy with concurrent chemotherapy for head and neck cancer. A 2026 randomized, double-blind, sham-controlled trial of an intraoral photobiomodulation device in head and neck radiotherapy provides direct trial confirmation. The main open questions are the exact wavelength, dose and treatment schedule, which vary between studies and clinics.

Low-level laser therapy for androgenetic alopecia has been tested in a number of randomized, sham-controlled trials, and a meta-analysis pooling them found a statistically significant increase in hair density with active treatment over sham. It is the best-supported of red light's appearance-related uses. The limitations are substantial: most trials ran for a few months, samples were small, follow-up was short, and a large share of the studies had manufacturer involvement, which tends to inflate apparent effects. It has not been compared head to head over the long term against the established treatments, topical minoxidil and, in men, oral finasteride.

In a prospective controlled trial, participants were treated with red or near-infrared light over roughly 30 sessions. Blinded evaluation of photographs and ultrasound assessment of dermal collagen showed improved skin complexion and self-reported skin feeling, reduction in fine lines, wrinkles and skin roughness, and an increase in intradermal collagen density. The two active light sources performed similarly. The design's main weakness is the absence of a blinded sham-light arm: the comparison was against an untreated, unmasked control group, not a placebo light, so part of the improvement could reflect expectation, and the sample was modest. This trial also had device-maker involvement, and skin trials in this field are frequently industry-linked.

Chronic lower-limb wounds, including venous and diabetic ulcers, heal slowly and are difficult to treat, so any additional lever is of interest. A systematic review of six randomized trials of LED photobiomodulation for these wounds concluded it may reduce wound area, improve the wound bed and increase local microcirculation, particularly in diabetic foot ulcers, but rated the overall certainty of the evidence very low. The benefit was concentrated at lower energy densities; the one trial using a very high dose (126 J/cm2) did not help and was associated with a longer healing time, in keeping with the biphasic dose window. The evidence base is limited by small samples, wide variation in wavelength, irradiance and total dose, and by an inability to pool the trials, which also leaves the ideal protocol unclear. It is applied clinically as an adjunct to standard wound care, not a replacement for it.

A systematic review and meta-analysis of randomized placebo-controlled trials examined low-level laser therapy for knee osteoarthritis pain and disability. Pooled across the trials, active treatment reduced pain compared with placebo, on the order of 15 mm on a 100 mm visual analog scale at the end of the treatment course, with relief persisting for some weeks afterward. The effect was strongly dose-dependent: trials delivering an adequate dose showed clear benefit, while under-dosed trials showed little to none, which explains much of the mixed reputation of the therapy for joint pain. Heterogeneity between protocols remains the main limitation.

A systematic review and meta-analysis published in a major medical journal pooled randomized placebo and active-treatment controlled trials of low-level laser therapy for neck pain. Active treatment produced greater pain relief than placebo, and in chronic neck pain the benefit was maintained for up to 22 weeks after the end of a treatment course, which is longer than many passive treatments manage. As with other pain uses, the trials were mostly short, and wavelength and dose differed between them, so the durability beyond that point and the best protocol are less certain.

A systematic review with meta-analysis assessed low-level laser therapy across tendinopathies. The pooled picture was mixed, but a consistent finding was that outcomes depended strongly on treatment parameters: trials using wavelengths and doses within a recommended window tended to show reduced pain and improved function, while trials outside that window tended to show little effect. This parameter sensitivity is the likely reason the therapy has an uneven reputation for tendon problems. Sample sizes were generally small and the tendons studied varied, so the strength of benefit for any one tendinopathy is uncertain.

A systematic review with meta-analysis pooled trials of phototherapy applied to muscle before exercise. Active treatment improved exercise performance measures (such as repetitions and time to exhaustion) and lowered post-exercise markers of fatigue and muscle damage, including creatine kinase, relative to placebo. Effect sizes were small to moderate. The results depend on the light being applied before, not after, exercise, and on wavelength and dose. A notable caveat is that a large fraction of the trials originate from a single research group, so independent replication across labs is what would move this from emerging toward established.

A systematic review of low-level laser therapy for reducing body circumferences pooled short trials, mostly in the abdomen, hips and thighs. It found modest reductions in measured circumference, typically a few centimeters, over treatment courses of a few weeks. The important limits: circumference is a tape-measure reading, not a measurement of fat mass, and can shift for reasons unrelated to fat; the trials were short with no long-term follow-up; and this is a commercially active area where many studies have manufacturer involvement. There is no good evidence that these devices produce durable fat loss or weight change, which is the claim marketing tends to imply.

Photobiomodulation is proposed to work through the absorption of red and near-infrared photons by chromophores in the cell, chiefly cytochrome c oxidase, complex IV of the mitochondrial electron transport chain. The absorbed light is thought to transiently raise mitochondrial membrane potential and ATP production and to modulate reactive oxygen species and nitric oxide, triggering downstream signaling that affects inflammation, blood flow and tissue repair. This account, developed largely from in-vitro and animal studies, explains why efficacy is wavelength-specific: the target chromophore absorbs in defined bands. It remains the working model, not a fully proven pathway, and the step from this cellular mechanism to a measured clinical effect is exactly what the trial evidence, claim by claim, has to establish.

A central and repeatedly observed feature of photobiomodulation is a biphasic, or bell-shaped, dose response, sometimes described by the Arndt-Schulz rule. Below a threshold dose there is no effect; within an effective window there is a beneficial response; above it the response diminishes and can reverse, becoming inhibitory. This has been documented across cell and animal studies and is a leading explanation for why trials with poorly chosen parameters fail while well-dosed trials succeed. The practical consequence is that the outcome depends on the combination of wavelength, irradiance (power per area), distance and exposure time, not on how strong a device sounds; more is not better, and past the window it can be worse.