Red Light Therapy Side Effects: What to Expect

Red light therapy is one of the safest therapeutic modalities studied in clinical research. Hundreds of randomised controlled trials have been conducted, and serious adverse events are vanishingly rare. But “very safe” is not the same as “no side effects at all.”

Understanding what you might experience — and why — helps you distinguish a normal response from something that needs attention. This guide covers every reported side effect, from the common and harmless to the rare and concerning, all grounded in the published clinical literature.

Common Side Effects

The following side effects are reported frequently by users and are considered normal physiological responses. They are typically mild and resolve within hours.

Temporary Skin Redness (Erythema)

The most commonly reported side effect is mild redness at the treatment site, appearing during or shortly after a session. This is not a burn — it is a vasodilatory response. Red and near-infrared light trigger the release of nitric oxide from endothelial cells, causing blood vessels to dilate and increasing local blood flow (Mittermayr et al., 2007, Free Radical Biology and Medicine).

This redness typically fades within 30–60 minutes. It looks and feels similar to the flush you get from exercise or a warm bath. If the redness persists beyond a few hours, you may be overdosing — reduce your session time or increase your distance from the device.

Warmth at the Treatment Site

You will feel warmth during treatment. This comes from two sources: the device itself generates some heat (LEDs are not perfectly efficient and emit thermal energy), and increased blood flow to the area produces a sensation of warmth.

This is entirely normal and generally pleasant. However, if the warmth becomes uncomfortable or progresses to a burning sensation, stop the session immediately. This is more common with contact devices (masks, wraps, pads) than with panels used at a distance.

Mild Headache

Some users report mild headaches during the first few sessions, particularly when treating the face, forehead, or scalp. Several mechanisms may explain this:

Increased nitric oxide — NO is a potent vasodilator, and rapid changes in cranial blood flow can trigger headaches in sensitive individuals
Eye strain — if you are not wearing goggles and are squinting against bright light
Detoxification response — increased cellular metabolism and circulation may temporarily mobilise metabolic waste products

Headaches typically resolve after the first week of use as your body adapts. If they persist, reduce session duration and ensure you are wearing appropriate eye protection. For transcranial applications, Salehpour et al. (2018, Molecular Neurobiology) noted that transient headaches occurred in a small percentage of participants but did not require treatment cessation.

Initial Skin Breakout (Purging)

Users treating acne or other inflammatory skin conditions sometimes experience a temporary worsening in the first 1–3 weeks. This “purging” phenomenon occurs because photobiomodulation accelerates cellular turnover and increases circulation to the skin, potentially bringing existing subclinical lesions to the surface faster.

This is analogous to the purging phase seen with retinoids and other active skincare ingredients. It is not a sign that the treatment is harming you — it is accelerating a process that would have happened anyway, just more slowly.

How to distinguish purging from a genuine adverse reaction:

Purging occurs in areas where you normally break out, peaks at 1–3 weeks, and then improves
A true reaction occurs in areas where you do not normally break out, worsens progressively, and does not resolve

If breakouts worsen beyond 3–4 weeks or spread to new areas, stop treatment and reassess.

Tightness or Dryness

Some users notice temporary skin tightness or dryness after sessions, particularly in the first few weeks. Red light can increase transepidermal water loss (TEWL) acutely by warming the skin surface. Applying a hydrating serum (hyaluronic acid is ideal) immediately after treatment addresses this effectively.

Rare Side Effects

The following side effects are uncommon but documented. They typically indicate either a dosing error, a device issue, or an individual sensitivity.

Burns

True burns from red light therapy are rare but not impossible. They occur almost exclusively with:

Contact devices (masks, wraps, pads) that trap heat against the skin. Prolonged contact with a device generating significant thermal output can cause superficial burns, particularly in users who fall asleep during treatment.
Exceptionally high-powered devices used at very close range for extended periods.

Burns are not caused by the red or NIR wavelengths themselves at therapeutic doses — the photon energy is far too low to cause thermal damage. They are caused by waste heat from the device hardware.

Prevention: Follow manufacturer guidelines for session length. Do not fall asleep with contact devices. If a device feels uncomfortably hot, remove it or increase distance.

Eye Strain and Potential Retinal Damage

Red light is visible and can cause eye strain if you stare at a bright panel without protection. Near-infrared light poses a more serious risk because it is invisible — your pupil does not constrict in response, and your blink reflex is not triggered. High-intensity NIR can damage the retina without any warning sensation.

The risk of permanent eye damage from home-use devices is low but not zero, particularly with powerful panels delivering >100 mW/cm² of NIR at close range. Clinical studies using transcranial NIR routinely require eye protection (Naeser et al., 2014, Photomedicine and Laser Surgery).

Prevention: Wear wavelength-specific goggles whenever your eyes face a device emitting NIR. See our eye protection guide for recommendations.

Fatigue and Malaise

A subset of users report feeling unusually tired after sessions, particularly in the first 1–2 weeks. This is sometimes described as a “detox” response, though the more precise explanation involves:

Increased metabolic demand — upregulated mitochondrial activity requires more substrate (glucose, oxygen)
Anti-inflammatory cytokine release — PBM modulates the immune system, which can cause transient fatigue similar to the mild malaise felt after a vaccination
Parasympathetic activation — some evidence suggests PBM can shift autonomic balance towards the parasympathetic (“rest and digest”) nervous system

This typically self-resolves within a week. If fatigue is severe or persistent, reduce treatment frequency or dose.

Insomnia or Sleep Disruption

Though less common, some users find that evening sessions leave them feeling alert and struggle to sleep. This is likely related to increased mitochondrial ATP production and potentially to modulation of melatonin pathways — though a 2019 study (Zhao et al., Frontiers in Neurology) found no significant sleep disruption from evening transcranial NIR.

Prevention: If you notice sleep disruption, move your sessions to the morning.

Hyperpigmentation in Darker Skin

There are anecdotal reports of temporary hyperpigmentation in individuals with darker skin tones (Fitzpatrick types IV–VI). This may relate to melanocyte stimulation from increased light exposure, though the mechanism is not well characterised in the PBM literature.

Kim et al. (2015, Lasers in Medical Science) noted that post-inflammatory hyperpigmentation was rare but possible in darker skin. Start with lower doses and shorter sessions, and monitor your skin response carefully. See our dark skin guide for specific protocols.

Why Side Effects Occur: The Biological Mechanisms

Understanding why side effects happen helps you respond appropriately rather than panicking.

The Herxheimer-Like Response

When PBM increases circulation and cellular metabolism, it can temporarily mobilise waste products, inflammatory mediators, and cellular debris faster than the body clears them. This is similar (though not identical) to the Jarisch-Herxheimer reaction seen when treating infections — the increased die-off of pathogens releases toxins that cause temporary symptoms.

In the PBM context, this manifests as fatigue, mild headache, skin purging, or generalised malaise during the first week or two of treatment. It is a sign that the therapy is bioactive, not that it is harmful.

Increased Circulation

Many side effects stem directly from vasodilation and increased blood flow. Nitric oxide release, a primary mechanism of PBM (Hamblin, 2017, BBA - General Subjects), causes vessels to relax and blood flow to increase. This is therapeutically desirable but can cause temporary flushing, warmth, headache, and even dizziness in sensitive individuals.

Mitochondrial Upregulation

PBM’s core mechanism — absorption of photons by cytochrome c oxidase in the mitochondrial electron transport chain — increases ATP production and generates a temporary burst of reactive oxygen species (ROS). At therapeutic doses, this ROS signalling activates beneficial pathways (Nrf2, NF-kB modulation). At excessive doses, it can overwhelm cellular antioxidant defences, leading to oxidative stress.

This is the cellular basis of the biphasic dose response — and why overdosing can cause effects that underdosing does not.

Side Effects by Device Type

Different devices carry different risk profiles.

LED Panels

Risk level: Low

Panels used at 15–60 cm distance pose minimal risk of burns (no skin contact) and allow easy dose control by adjusting distance and time. The primary risks are eye exposure (use goggles) and overdosing if used at close range for extended periods.

LED Face Masks

Risk level: Low–Moderate

Masks sit in direct contact with facial skin. Most consumer masks use relatively low irradiance (5–30 mW/cm²) and include automatic timers, making overdosing unlikely. However:

Heat can build up under the mask, causing discomfort or redness
Masks that cover the eyes require integrated eye shielding — check that yours provides it
Contact with the skin means any heat generated transfers directly; remove immediately if uncomfortable

Wraps and Pads

Risk level: Low–Moderate

Similar risk profile to masks. Contact devices that wrap around joints or limbs can trap heat. The risk of superficial burns increases if you fall asleep during treatment or ignore discomfort. Always use the device’s built-in timer and do not exceed recommended session lengths.

Laser Devices

Risk level: Moderate

Class 3B and Class 4 lasers used in clinical settings deliver concentrated beams at much higher irradiances than LED panels. Eye protection is absolutely mandatory. Burns are possible if the beam is held stationary on one spot for too long. These devices should only be used by trained practitioners.

Home-use laser devices (typically Class 2 or low-end Class 3B, like the Kineon Move+) are designed with safety features to prevent harm, but laser-specific goggles are still essential.

When to Stop Treatment

Discontinue your session immediately and reassess your protocol if you experience:

Burning sensation — not warmth, but actual pain. This indicates thermal injury risk.
Blistering or skin damage — stop treatment, allow the area to heal completely before resuming.
Persistent headache — lasting more than a few hours post-treatment.
Worsening of the condition being treated — beyond the initial 2–3 week adjustment period.
Unusual rash or hives — may indicate a photosensitivity reaction, possibly medication-related.

Discontinue entirely and consult a doctor if:

You develop a rash that spreads or does not resolve
You experience visual disturbances after treatment
Side effects worsen over multiple sessions rather than improving
You have started a new medication and notice increased skin sensitivity (see our medications guide and contraindications page)

Rash, Itching, and Nausea: What They Mean

Rash or Itching

A localised rash or itching at the treatment site may indicate:

Contact irritation — from a mask or wrap material, not the light itself
Heat rash — from trapped warmth under contact devices
Photosensitivity reaction — particularly if you are taking photosensitising medications (tetracyclines, doxycycline, certain diuretics, St. John’s wort)
Allergic contact dermatitis — to silicone, adhesive, or other device materials

If the rash occurs only under a contact device, try a session with a panel at distance. If it resolves, the issue is the device material, not the light.

Nausea

Nausea during or after red light therapy is uncommon but reported. Possible explanations include:

Vagal response — the heat and vasodilation may trigger a vasovagal response in susceptible individuals
Detoxification response — as discussed above
Anxiety or claustrophobia — particularly with full-face masks

If nausea occurs, sit or lie down, hydrate, and reduce the dose at your next session.

Risk Factors: Who Is More Likely to Experience Side Effects?

Certain factors increase your likelihood of experiencing side effects:

Photosensitising medications — the most significant risk factor. Doxycycline, tetracycline, hydrochlorothiazide, amiodarone, methotrexate, and many others increase skin sensitivity to light. Check our medications page and consult your prescriber.
Fair skin — Fitzpatrick types I–II are more prone to erythema and heat sensitivity
Active inflammatory skin conditions — eczema, rosacea, and psoriasis may flare initially before improving
First-time users — side effects are most common in the first 1–2 weeks and typically diminish with continued use
High-dose protocols — users who start with aggressive dosing are more likely to experience the biphasic overdose response

How to Minimise Side Effects

Start low, build gradually. Begin at the lower end of recommended doses (e.g., 3–5 J/cm² for skin, 10 J/cm² for deep tissue). Increase by 10–20% per week if well tolerated.
Check your medications. Review the medications page before starting. If you take any photosensitiser, discuss RLT with your prescriber.
Wear eye protection. Every session where your eyes face the device. Non-negotiable.
Use built-in timers. Do not override automatic shut-off features on masks and wraps.
Hydrate. Drink water before and after sessions. Increased circulation and metabolic activity benefit from adequate hydration.
Keep a session log. Record duration, distance, any side effects. Patterns become visible quickly and guide your adjustments.
Do not combine with other light-based treatments on the same day without professional guidance (IPL, laser resurfacing, UV therapy).

Long-Term Safety: Is There a Cumulative Risk?

A common concern among new users is whether long-term, repeated exposure to red and near-infrared light could cause cumulative damage — similar to how chronic UV exposure damages skin over decades.

The answer, based on current evidence, is reassuring. Red and NIR photons carry far less energy than UV photons. UV light (particularly UVB at 280–315 nm) causes direct DNA damage through pyrimidine dimer formation. Red light at 630–660 nm carries approximately half the photon energy of UVB — well below the threshold for DNA damage.

No long-term studies have identified cumulative adverse effects from repeated PBM exposure. Workers in industries with high red/NIR light exposure (such as certain manufacturing environments) have not shown elevated rates of skin disease. This is consistent with the basic photophysics: photon energy at these wavelengths is simply insufficient to break molecular bonds in DNA or proteins.

The theoretical concern about chronic mitochondrial overstimulation exists in the literature (Karu, 2010, Photochemistry and Photobiology), but this would require sustained, extreme doses far beyond any reasonable home-use protocol. Rest days between sessions provide ample recovery time for normal cellular homeostasis to re-establish.

The bottom line: Daily use of red light therapy at recommended doses appears safe over months and years. The biological mechanisms do not support a cumulative damage model at therapeutic intensities.

Interactions with Other Light-Based Treatments

If you use other light-based therapies or procedures, timing matters:

IPL (intense pulsed light) — do not use RLT on the same area on the same day. IPL causes controlled photodamage; adding PBM immediately could alter the inflammatory response. Wait 48–72 hours.
Laser skin resurfacing — wait until the acute healing phase (typically 5–7 days) before resuming RLT. After the acute phase, PBM may actually accelerate recovery.
UV phototherapy (narrowband UVB for psoriasis, eczema) — RLT and UV therapy on the same day is generally acceptable as they work through entirely different mechanisms. However, UV increases skin sensitivity, so reduce your RLT dose on UV therapy days.
Blue light therapy for acne — this combines well with red light. Many clinical trials use blue + red sequentially in the same session with excellent safety profiles.

What the Clinical Literature Reports

The adverse event data from RCTs is reassuring. Across the PBM literature:

Wunsch & Matuschka (2014) — 136 volunteers received facial LED treatments over 30 sessions. No significant adverse events. Mild and transient erythema was the only noted effect.
Avci et al. (2013), reviewing multiple dermatological RCTs, concluded that “LED phototherapy appears to be safe, with no reports of serious adverse effects.”
Heiskanen & Hamblin (2018) published a comprehensive safety review in Photobiomodulation, Photomedicine, and Laser Surgery, finding that adverse events in controlled studies were “rare, mild, and self-limiting.”
Lanzafame et al. (2014), in hair growth trials, reported no adverse events across 146 subjects over 26 weeks.
Salehpour et al. (2018), reviewing transcranial PBM, noted occasional mild headache but no serious adverse events across multiple trials.

The most significant risks identified across the literature are not from the light itself but from device-related issues (burns from contact heat, eye exposure) and drug interactions (photosensitising medications).

Summary

Red light therapy has an excellent safety profile supported by extensive clinical data. The side effects that do occur are typically:

Mild and transient (redness, warmth, headache)
Most common in the first 1–2 weeks of use
Manageable by adjusting dose, distance, and frequency
More likely in users taking photosensitising medications

Serious adverse events are rare and almost always preventable through proper eye protection, sensible dosing, and awareness of contraindications.

Start conservatively, monitor your response, and adjust methodically. If something feels wrong, reduce the dose before increasing it. And if you take any medication, check the interactions list before your first session.

This article is for educational purposes only and does not constitute medical advice. If you experience persistent or concerning side effects, consult a healthcare professional. See our contraindications guide for conditions that may preclude use.