The side effects of laser therapy in physiotherapy are mild and temporary, and most patients have none at all. The most common ones are a gentle warming sensation during treatment, slight redness in the treated area, mild soreness like the feeling after exercise, and occasional short-lived fatigue. These effects usually fade within a few hours to a couple of days, and serious complications are rare when a trained practitioner delivers the treatment.

Below we explain what laser therapy in physiotherapy is, whether it works, and every side effect you might notice. We also cover how Class IV and cold laser differ, side effects specific to knee pain, who should avoid laser therapy, which medications matter, how to prepare, how you feel afterward, and how many sessions you can expect.

What Is Laser Therapy in Physiotherapy?

Laser therapy in physiotherapy is a treatment that uses focused light energy to reduce pain, calm inflammation, and speed up healing in muscles, tendons, ligaments, and joints. This light energy is delivered through a handheld device moved over the skin, and it reaches the tissue underneath without any cutting or heat damage. The technical name for this process is photobiomodulation, and you may also hear it called low-level laser therapy (LLLT) or Class IV laser therapy.

Photobiomodulation works by sending specific wavelengths of light, in the red to near-infrared range of 600 to 1000 nanometres (nm), into the deeper layers of tissue. These wavelengths are absorbed by your cells, where they boost the production of cellular energy called adenosine triphosphate (ATP), increase local blood flow, and reduce inflammation. Higher cellular energy and better circulation together speed the repair of damaged tissue, which is why laser therapy is used across physiotherapy and chiropractic care for both fresh injuries and long-standing pain.

The conditions treated with laser therapy include chronic low back and neck pain, tendonitis, bursitis, plantar fasciitis, sciatica, arthritis, and sports injuries. Because the treatment is non-invasive and drug-free, it fits easily alongside hands-on physiotherapy and exercise. Knowing what laser therapy is sets up the more important question for most patients, which is whether it actually delivers results.

Does Laser Therapy in Physiotherapy Actually Work?

Yes, laser therapy in physiotherapy works, and strong clinical research backs it for pain relief and faster healing. Research shows that laser therapy reduces pain, lowers inflammation, and improves function across a range of musculoskeletal conditions. These results explain why laser therapy is a trusted tool despite its handful of mild side effects.

The evidence is substantial. A 2026 systematic review published in Springer pooled 53 studies covering roughly 2,800 patients and found that laser therapy cut average pain scores on the visual analogue scale by 32% compared with control treatment. The same review reported that laser therapy improved muscle strength outcomes by 25% and reduced delayed-onset muscle soreness by 30% when paired with exercise. A landmark meta-analysis in The Lancet, drawing on 16 randomized controlled trials and 820 patients, found that people with chronic neck pain were about four times more likely to improve with laser therapy than with a placebo.

Laser therapy also produces lasting relief, not just a quick fix. In that same Lancet analysis, pain relief persisted for up to 22 weeks after treatment ended, and pain intensity dropped by nearly 20 millimetres on a standard 100-millimetre pain scale. Does laser therapy speed up healing? Yes, laser therapy speeds up healing by increasing cellular energy and circulation, which shortens recovery time for many injuries. Since laser therapy clearly works, the practical concern becomes what side effects come with it.

What Are the Side Effects of Laser Therapy in Physiotherapy?

The side effects of laser therapy in physiotherapy are mild, temporary, and uncommon, and they usually come from the body’s own healing response rather than from any harm. Most patients finish a session feeling nothing unusual at all. When side effects do appear, they are gentle and short-lived, and they resolve on their own without treatment.

The possible side effects of laser therapy include:

• Mild warmth: A gentle warming sensation in the treated area during the session, caused by increased blood flow. This feels similar to a warm compress and fades quickly.
• Temporary redness: Slight redness of the skin afterward from improved circulation and oxygen delivery, which typically clears within a few hours.
• Mild soreness: A dull ache similar to the feeling after a workout, caused by the healing response switching on. Soreness usually settles within 24 to 48 hours.
• Temporary fatigue: Occasional tiredness as the body directs energy toward tissue repair, which rest and water resolve quickly.
• Localized sensitivity: The treated spot may feel slightly tender for a short time, easing as healing continues.

What are two drawbacks of laser therapy? The two most common drawbacks are mild soreness after a session and the need for several visits to feel the full benefit, since laser therapy builds results over a course of treatment rather than in one visit. Reassuringly, a large meta-analysis in The Lancet found that side effects from laser therapy were mild and no different from those reported in the placebo group. Knowing the side effects are minor, many patients still want to know whether the strength of the device changes the picture.

Are Class IV Laser Side Effects Different From Cold Laser?

Class IV laser side effects are largely the same as cold laser side effects, though a Class IV laser produces a stronger warming sensation because it uses higher power. Both types are non-invasive and share the same mild, temporary profile of warmth, redness, and short-lived soreness. The main difference lies in power and how deep the light reaches, not in the type of side effect you might feel.

A Class IV laser, like the K-Laser used at our Markham clinic, delivers higher-power wavelengths that penetrate deeper into muscle, tendon, and joint tissue, which lets it treat larger areas in less time. Cold laser, also called low-level laser therapy, is gentler and works on more superficial tissue. Because a Class IV device carries more energy, the warming sensation is more noticeable, and careful dosing matters more, which is exactly why a trained practitioner controls the settings for your condition during K-Laser treatment. The table below compares the two.

Feature	Class IV Laser (K-Laser)	Cold Laser (Low-Level Laser)
Power level	High power, deeper energy	Low power, gentle energy
Tissue depth reached	Deep muscle, tendon, joint	More superficial tissue
Session length per area	5 to 15 minutes	Often longer for the same area
Warming sensation	More noticeable warmth	Little to no warmth
Side-effect profile	Mild and temporary	Mild and temporary
Best suited for	Chronic and deep conditions	Surface-level conditions

Sources: Proback Clinic; KC Rehab clinical information; Springer systematic review (2026).

Both lasers stay well within a mild side-effect range, so the choice comes down to how deep your condition sits rather than to safety differences. One of the most common areas we treat with a Class IV laser is the knee, which raises its own specific questions.

What Are the Side Effects of Laser Therapy for Knee Pain?

The side effects of laser therapy for knee pain are the same mild, temporary effects seen elsewhere on the body, most often brief warmth and slight soreness around the knee. The knee is a shallow joint with little fat covering it, so patients sometimes notice the warming sensation a little more clearly here. Even so, these effects stay minor and fade within a day or two.

Laser therapy is a popular choice for knee conditions like osteoarthritis, tendonitis, and ligament strains because it reaches the joint without injections or medication. Occasionally the knee may feel slightly tender or warm for a few hours after a session, which reflects the increased circulation working to repair the joint tissue. Anyone managing ongoing knee pain can usually continue their normal activity right after treatment.

If you are already doing strengthening work through exercise rehabilitation, laser therapy pairs well with it and rarely adds more than a passing ache. Combining the two often shortens recovery and keeps flare-ups down between visits. With the side effects clear across the body and the knee, the next natural question is how safe laser therapy is overall.

Is Laser Therapy in Physiotherapy Safe?

Yes, laser therapy in physiotherapy is safe, and it is one of the lowest-risk treatments available for musculoskeletal pain. Safety comes from the fact that therapeutic laser light stimulates cells without cutting, burning, or damaging tissue. Like other gentle physiotherapy modalities such as ultrasound therapy, it works with the body’s own repair process rather than against it. When a qualified practitioner delivers it, the treatment carries very little risk and needs no recovery downtime.

The safety record is well documented. The 2026 Springer systematic review of 53 studies reported no serious adverse events across roughly 2,800 patients, and The Lancet meta-analysis confirmed that side effects were no worse than placebo. In Canada, Class IV laser therapy is Health Canada-approved for musculoskeletal pain and inflammation, and at our clinic Class IV K-Laser therapy is delivered only by practitioners regulated by their provincial colleges, including the College of Physiotherapists of Ontario and the College of Chiropractors of Ontario. Eye protection is always worn by both the patient and the practitioner, since direct eye exposure to laser light is the one meaningful risk, and it is easily prevented. Is laser therapy safe long term? Yes, laser therapy is safe for long-term use because it does not build up in the body or cause cumulative tissue damage. Safe as it is for most people, laser therapy is not right for everyone.

Who Should Not Do Laser Therapy?

People who should not do laser therapy include anyone with active cancer in the treatment area, pregnant patients when treating the abdomen or lower back, and those with light-sensitive skin conditions. These situations call for caution because laser light stimulates cell activity and circulation, which is not appropriate over certain tissues. A clinical assessment before treatment identifies whether laser therapy is safe for you.

The main situations where laser therapy is avoided or modified are:

• Active cancer or a known tumour in the area to be treated, because laser stimulates cell growth and circulation.
• Pregnancy, particularly over the abdomen, pelvis, or lower back.
• Photosensitive skin conditions that react strongly to light.
• Treatment directly over the eyes or the thyroid gland.
• Areas with active bleeding or an untreated infection.

This list is not a reason for worry, since most people have none of these conditions and can receive laser therapy safely. A trained practitioner reviews your full health history first and will recommend a different approach if laser is not suitable. One part of that history that deserves its own attention is the medication you take.

What Medications Can’t You Take With Laser Therapy?

The medications you should be cautious about with laser therapy are those that make your skin more sensitive to light, known as photosensitizing drugs. These medications can raise the chance of skin redness or irritation in the treated area. Telling your practitioner about every medication you take lets them adjust the treatment or choose an alternative.

Photosensitizing medications include certain antibiotics such as tetracyclines, some acne treatments like isotretinoin, particular diuretics, and certain medications used in cancer care. Some patients taking steroid medications or blood thinners are also assessed more carefully before treatment. None of these automatically rule out laser therapy, but they do call for a careful review and sometimes a lower dose. Because this assessment happens during your first visit, the next thing worth knowing is how to prepare for a session.

What Should You Not Do Before Laser Treatment?

Before laser treatment you should not apply lotions, oils, or creams to the treatment area, and you should avoid heavy sun exposure or tanning on that skin. Products on the skin can interfere with how the light is absorbed, and sun-sensitized skin may react more strongly. Arriving with clean, product-free skin helps the treatment work as intended.

A few simple steps make each session smoother. Wear loose clothing that gives easy access to the area being treated, and let your practitioner know if you have started any new medication since your last visit. You do not need to fast, rest, or change your routine beforehand, since laser therapy requires no special preparation beyond clean skin. With preparation handled, most patients want to know what to expect once the session is done.

How Do You Feel After Laser Therapy, and What Should You Do?

After laser therapy most people feel normal, and some feel immediate relief in the treated area, while a few notice mild warmth, soreness, or tiredness. These sensations are gentle signs that the healing response has started. There is no downtime, so you can return to your usual activities right away.

How long does it take to recover from laser therapy? Recovery from laser therapy is immediate for most patients, and there is no required rest period afterward. How many days of rest are required after laser treatment? Zero days of rest are required after laser treatment, because the therapy is non-invasive and you can drive, work, and exercise the same day. To get the most from a session, drink plenty of water to support your body’s repair process, and if you feel mild soreness, treat it like post-exercise soreness and let it settle over a day or two. Does laser therapy make you tired? Laser therapy makes a small number of patients feel briefly tired as the body uses energy for tissue repair, and this passes quickly with rest and hydration. Knowing what to expect after a session leads naturally to the bigger picture of how many sessions a full course involves.

How Many Laser Therapy Sessions Are Needed, and How Long Do They Take?

Most people need between four and ten laser therapy sessions, and each session lasts about 5 to 15 minutes per treatment area. The exact number depends on how severe your condition is and how long you have had it. Acute injuries usually respond faster than long-standing chronic problems.

A typical course of laser therapy follows this pattern:

1. Acute conditions: often 3 to 6 sessions for recent injuries and flare-ups.
2. Moderate conditions: around 6 to 10 sessions for conditions that have lasted several weeks.
3. Chronic conditions: 10 to 15 or more sessions for pain that has persisted for months.

Sessions are usually spaced a few days apart at first, then tapered as your symptoms improve. Because each visit is short and needs no recovery time, laser therapy fits easily into a busy schedule at our Markham clinic and slots neatly alongside other treatments like acupuncture or hands-on care. Your practitioner gives you a clear estimate after the first assessment and adjusts it based on how your body responds. Knowing the full picture of side effects, safety, and sessions makes it easier to decide whether laser therapy is right for you.

Frequently Asked Questions

Is Laser Therapy Risky?

Laser therapy is not risky for most people, and it is considered one of the safest treatments for musculoskeletal pain. The only notable risk is direct eye exposure to the laser light, which is fully prevented by the protective eyewear worn during every session. Serious complications are extremely rare when a trained practitioner delivers the treatment.

Does Laser Therapy Make You Tired?

Laser therapy makes only a small number of patients feel mildly tired afterward, as the body directs energy toward tissue repair. This tiredness is temporary and usually resolves within a few hours with rest and water. Most patients feel completely normal and notice no fatigue at all.

How Long Does It Take to Recover From Laser Therapy?

Recovery from laser therapy is immediate, because the treatment is non-invasive and requires no downtime. You can return to work, driving, and exercise the same day. If mild soreness appears, it settles on its own within 24 to 48 hours.

What Are Two Drawbacks of Laser Therapy?

The two main drawbacks of laser therapy are that it can cause brief soreness after a session and that it usually takes several visits to reach full benefit. Laser therapy builds results over a course of treatment rather than in a single visit. Both drawbacks are minor compared with the treatment’s strong safety and pain-relief record.

How Long Is a Physiotherapy Laser Therapy Session?

A physiotherapy laser therapy session lasts about 5 to 15 minutes per treatment area. Larger or multiple areas take a little longer, but the treatment is still brief. There is no recovery time afterward, so you can carry on with your day right away.

Does Laser Therapy Speed Up Healing?

Yes, laser therapy speeds up healing by boosting cellular energy production and increasing blood flow to injured tissue. Research pooling 53 studies found laser therapy reduced pain by an average of 32% and improved recovery when combined with exercise. Faster cellular repair is the reason many athletes and active adults use it.

Understanding the Side Effects of Laser Therapy

Laser therapy in physiotherapy is a safe, effective, and low-risk treatment whose side effects are mild and short-lived. The most you are likely to feel is a little warmth during the session, slight redness, mild soreness, or brief tiredness, all of which fade within hours to a couple of days. Strong research confirms that laser therapy relieves pain and speeds healing while causing side effects no worse than a placebo, and it needs no recovery downtime.

If you are dealing with chronic pain, a sports injury, or stubborn inflammation, laser therapy may be a comfortable and drug-free way to recover faster. The team at KC Rehab will assess your condition, review your health history, and let you know whether K-Laser treatment is the right fit, and you can contact us whenever you are ready to start.