Laser therapy is designed to relieve pain, but under specific circumstances it can genuinely make pain worse. The reasons range from straightforward overheating of tissue to subtler biological responses where too much light energy flips a helpful process into a harmful one. Whether the problem is a dose that crosses a threshold, a wavelength that directly excites pain-sensing nerve channels, or even the patient’s own expectations amplifying pain signals, the answer is not a simple yes or no. It depends heavily on how the therapy is delivered and who is receiving it.
The Biphasic Dose Response
One of the most important concepts in laser therapy is the idea that the relationship between dose and benefit is not a straight upward line. At very low energy levels, nothing happens because the tissue never reaches the minimum threshold needed to trigger a biological response. As the dose increases past that threshold, cells respond positively: metabolism picks up, inflammation starts to calm down, and healing processes kick in. But keep increasing the energy, and the benefit curve peaks and then reverses. At high enough doses, the stimulation disappears and is replaced by inhibition, where the tissue actually performs worse than if you had done nothing at all.1PubMed Central. Biphasic Dose Response in Low Level Light Therapy
This pattern shows up at every level of biological organization, from individual cells to whole tissues to what patients report in the clinic. For someone receiving laser therapy, the practical implication is that more is not better. A clinician who doubles the treatment time or cranks up the power density thinking it will double the relief may push the dose past the beneficial window and into a zone where pain and inflammation actually worsen. The biphasic curve is not some exotic edge case; it is the expected behavior of living tissue exposed to light energy. Getting the dose right is the entire game.
When Heat Becomes the Problem
The most intuitive way laser therapy can worsen pain is plain overheating. All laser light that enters tissue eventually converts some of its energy to heat, and how much heat accumulates depends on the power level, the exposure time, and whether the beam is continuous or pulsed.
The difference between continuous wave and pulsed delivery is dramatic. In one comparison, a continuous-wave laser running at 30 watts raised skin surface temperature to roughly 76°C after five minutes of exposure, an increase of about 40°C that far exceeds any safe threshold for human tissue. A pulsed laser delivering the same peak power but cycling on and off at a 10 percent duty cycle raised the temperature by only about 2°C over the same period.2Journal of Biophotonics. Comparative Analysis of Pulsed and Continuous Wave Modes in High‐Intensity Laser Light Therapy: Implications for Deep Tissue Treatment That gap illustrates why delivery mode matters so much: continuous-wave lasers at high power can produce burns if applied carelessly, while pulsed modes let tissue cool between bursts.
Even at more moderate power levels, measurable warming occurs. In a pilot study of high-intensity laser therapy applied to the jaw area in horses, the treated region warmed by an average of about 2°C, with no adverse reactions observed at those settings.3MDPI Animals. Thermal Effects of High-Intensity Laser Therapy on the Temporomandibular Joint Area in Clinically Healthy Racehorses—A Pilot Study A couple of degrees is well within the tolerable range, and for many therapeutic applications a mild warming sensation is considered part of the treatment. The trouble starts when the temperature climbs past the point where tissue proteins begin to denature and nerves start firing pain signals, which happens rapidly at the extremes that improperly configured continuous-wave devices can reach.
Skin Tone and Absorption
Heat generation is not the same for everyone. Melanin, the pigment that determines skin color, is a powerful absorber of laser light, especially at shorter wavelengths. People with darker skin (Fitzpatrick types V and VI) absorb substantially more laser energy in the outer layers of their skin compared to people with lighter skin. That extra absorption translates directly into more heat, more pain during the procedure, and a greater risk of thermal injury.4PubMed Central. Laser Therapy in Ethnic Populations
This is well documented in the laser hair-removal literature, where patients with more melanin consistently report more discomfort even when cooling devices are used. The clinical solution is to lower the fluence and use more conservative settings, but that adjustment requires awareness. A device preset calibrated for lighter skin can deliver a painful, injury-prone dose to someone with darker skin. For therapeutic lasers used in physical therapy and pain management, the same physics applies: melanin does not care why the laser is being pointed at the skin.
Light That Directly Activates Pain Receptors
Heat is not the only way laser light produces a pain signal. Some wavelengths appear to directly excite pain-sensing channels in nerve cells without needing to warm tissue first. Research has shown that green laser light at 532 nanometers produces power-dependent activation of TRPV1, a receptor channel on sensory neurons that is best known for responding to capsaicin (the compound that makes chili peppers burn) and to temperatures above about 43°C.5PubMed Central. Stimulation of TRPV1 by Green Laser Light
The fact that a laser can open these channels at the cellular level matters because TRPV1 activation is one of the body’s primary pain-signaling pathways. If a therapeutic laser is operating at a wavelength or intensity that hits this channel, the patient may experience a stinging or burning sensation that is not caused by actual tissue damage but by the light itself telling the nervous system to feel pain. The practical relevance is limited because most therapeutic lasers operate at wavelengths in the red and near-infrared range (roughly 630 to 1,100 nanometers), well away from the 532 nanometer green light used in that research. But it demonstrates that the interaction between laser light and pain is more complex than “heat equals hurt.” Specific photon energies can talk directly to the molecular machinery of pain.
Oxidative Stress From High-Power Protocols
Beyond heat and direct nerve activation, laser therapy also affects pain through chemistry at the cellular level. The primary mechanism behind most of laser therapy’s benefits involves a mitochondrial enzyme that absorbs light and responds by ramping up energy production. Part of what makes this work is that light knocks a molecule of nitric oxide off the enzyme, allowing the mitochondria to consume more oxygen and produce more of the cell’s energy currency.6PubMed Central. Mechanisms and applications of the anti-inflammatory effects of photobiomodulation At the right dose, this cascade reduces inflammation and supports tissue repair.
But increased mitochondrial activity also produces reactive oxygen species, which are chemically aggressive molecules that can damage cells if they accumulate. In a study of microglial cells (immune cells in the brain) exposed to high-power laser protocols, all irradiation protocols tested produced a significant increase in reactive oxygen species. Some protocols showed levels that remained elevated above normal for the entire 24-hour observation period.7PubMed Central. High-Power Laser Therapy Modulates Mitochondrial Function and Redox Balance Without Cytotoxicity: An In Vitro Study in BV-2 Microglial Cells While that particular study did not find outright cell death, persistently elevated oxidative stress is a known driver of inflammation and tissue irritation. In a clinical setting, pushing the dose high enough to chronically elevate these reactive molecules could plausibly counteract the anti-inflammatory benefits the therapy was meant to provide, and contribute to a flare of discomfort.
This connects back to the biphasic dose response discussed earlier. At moderate doses, the bump in reactive oxygen species serves as a beneficial signaling molecule that helps cells adapt. Past a certain point, the oxidative burden overwhelms the cell’s defenses, tipping the balance from healing toward harm.
How Expectations Shape the Pain Experience
Not all worsening of pain during laser therapy has a physical explanation rooted in tissue or cells. The nocebo effect, where a negative expectation produces a genuinely worse outcome, has been directly studied in the context of laser-induced pain. In one experiment, healthy volunteers were told that an inert cream applied to one hand would increase the pain they felt from a laser stimulus. Even though the cream did nothing, the mere verbal suggestion was enough to disrupt the normal habituation of pain. Ordinarily, repeated laser pulses at the same intensity feel progressively less painful as the brain tunes out the signal. After the nocebo suggestion, that habituation failed, and the subjects continued to rate the pain as high.8PubMed. Expectation to feel more pain disrupts the habituation of laser-pain rating and laser-evoked potential amplitudes
This was not just a matter of people saying the pain was worse while feeling the same thing internally. The brain’s electrical response to the laser pulses, measured by evoked potentials, also failed to habituate. The expectation to feel more pain changed the actual neural processing of the pain signal.9PubMed Central. Nocebo and pain: an overview of the psychoneurobiological mechanisms Related research has found that invalid cues about stimulus intensity can make low-intensity laser stimuli feel more painful than they objectively are, with measurable changes in cortical activity.10PubMed. Cortical correlates of false expectations during pain intensity judgments–a possible manifestation of placebo/nocebo cognitions
For anyone undergoing laser therapy who has been told by a friend that it hurts, or who read an alarming account online, or who simply arrives at the appointment feeling anxious, these findings are relevant. The expectation itself can amplify genuine pain signals, and the amplification is not imaginary. This does not mean the pain is “all in your head” in any dismissive sense. It means that the nervous system actively adjusts its gain based on context, and negative expectations turn that gain up. A clinician who takes a few minutes to explain what the therapy will feel like and reframe the expected sensation can influence the actual pain a patient experiences.
What Adverse Events Look Like in Clinical Trials
Given all the ways laser therapy could theoretically worsen pain, what do clinical trials actually report? The picture is reassuring, if a bit thin on detail. In a systematic review of laser and energy-based device therapies for surgical scars, only minor adverse effects were reported across the 17 included trials. In the low-level laser therapy groups specifically, no adverse events or significant pain were reported at all.11PubMed Central. Effect of Laser and Energy-based Device Therapies to Minimize Surgical Scar Formation: A Systematic Review and Network Meta-analysis
When laser therapy is compared to other treatments, it tends to come out looking relatively gentle. A systematic review and meta-analysis comparing extracorporeal shock wave therapy to laser therapy for musculoskeletal conditions found that the adverse effects documented in the literature (bruising, pain, temporary redness) were associated with the shock wave treatments, not the laser groups.12PubMed Central. Extracorporeal Shock Wave Therapy versus laser therapy in treating musculoskeletal disorders: a systematic review and meta-analysis That does not mean laser therapy never causes discomfort in practice, but it suggests that serious or lasting pain from laser therapy is uncommon in controlled settings where protocols are followed.
The gap between “what can happen” and “what typically happens” is worth understanding. Most clinical trials use carefully calibrated doses, trained operators, and population-appropriate settings. The risks described in earlier sections of this article, thermal injury from misconfigured continuous-wave devices, excessive oxidative stress from overdosing, pain amplified by melanin absorption, these are real failure modes, but they are failure modes that competent clinical practice is specifically designed to avoid. A post-treatment pain flare that lasts a few hours is reported anecdotally by some patients, particularly in the first session or two, and is generally considered part of the inflammatory response being temporarily upregulated before settling down. If pain steadily worsens over multiple sessions or is sharp and burning during treatment, something about the protocol likely needs adjustment.
Wavelength and Penetration Depth
The wavelength of the laser determines not only which biological targets it hits but how deep into tissue it travels. Longer wavelengths in the near-infrared range penetrate deeper than shorter wavelengths in the visible spectrum. A comparison of 905 nanometer and 1064 nanometer lasers found that the longer-wavelength device achieved greater penetration depth through both muscle and skin tissue, with the most pronounced differences in the upper 10 millimeters.13PubMed Central. Comparison of the Penetration Depth of 905 nm and 1064 nm Laser Light in Surface Layers of Biological Tissue Ex Vivo
This matters for the pain question because a laser that deposits most of its energy in the superficial layers of skin heats those layers more and penetrates to the target tissue less. A patient being treated for a deep joint or tendon problem with a wavelength that gets absorbed mostly in the top few millimeters of skin may feel more surface-level discomfort while getting less therapeutic benefit at the actual injury site. Conversely, a longer wavelength that passes through skin more easily delivers energy deeper with less surface heating, which tends to be more comfortable for the same therapeutic effect. Clinicians selecting wavelengths are balancing these tradeoffs, but patients who experience unexpected pain during treatment may simply be receiving a wavelength that is a poor match for the depth of their problem.
Home Devices and Unsupervised Use
The growing market for consumer laser therapy devices introduces another layer of risk. Devices marketed for home use range from low-power LED panels that are virtually incapable of causing harm to handheld class 3B and even class 4 lasers that deliver therapeutically meaningful power. The issue is not that these devices are inherently dangerous but that they come without a clinician adjusting settings for the individual patient.
Without professional guidance, a user may treat the same spot for too long, use the device on skin that has not been assessed for melanin-related absorption risk, or stack treatments too close together in the hope of faster results. All of these behaviors push the dose up the biphasic curve toward the inhibitory zone described earlier. A home user experiencing a pain flare after using a laser device may assume the treatment is not working and increase the dose further, which is exactly the wrong response. The general principle is simple: if a session makes you feel worse rather than better, the next step is to reduce the dose or frequency, not increase it. Pain that worsens progressively across sessions is a signal to stop and consult a professional, not to push through.
Eye safety is a separate but serious concern with home devices. Direct or reflected laser light entering the eye can cause retinal damage that is painless at the moment it occurs and permanent. Proper protective eyewear rated for the specific wavelength in use is non-negotiable for any laser with therapeutic power output, and this applies equally to bystanders and pets in the room.
Temporary Flares Versus Genuine Worsening
One of the most common sources of confusion for people undergoing laser therapy is the temporary flare. In the first one to three sessions, some patients report that their pain increases for several hours or even a day before settling to a level below where it started. This pattern is consistent with the inflammatory cascade being temporarily upregulated by the treatment. Laser therapy at appropriate doses promotes the release of signaling molecules that recruit immune cells and increase blood flow to the area, processes that are part of healing but feel like inflammation in the short term.
A temporary flare that resolves within 24 to 48 hours and gives way to progressive improvement is generally not a sign that the therapy is making things worse. A flare that intensifies with each session, or that lasts longer than a couple of days, is a different signal. It suggests the dose is too high, the wavelength is wrong for the condition, or the underlying problem is not one that laser therapy can help. Some conditions involving active infection or malignancy are contraindicated for laser therapy entirely, not because the laser worsens those conditions through the dose-response mechanisms described above, but because increasing blood flow and cellular activity in those tissues can accelerate a process you want to slow down.
The distinction between “it hurts a bit more today but I’m trending better” and “it’s getting steadily worse” is the single most useful thing a patient can track. Keeping a simple pain diary with a numerical rating before and after each session, plus a daily score on non-treatment days, gives both the patient and clinician the information they need to decide whether the protocol is helping or needs to change.