Ultraviolet light therapy, also called phototherapy, is a medical treatment that uses specific wavelengths of UV light to treat skin conditions like psoriasis, eczema, and vitiligo. It works by calming overactive immune responses in the skin, slowing the rapid cell growth that causes plaques and rashes. Most courses of treatment run 20 to 36 sessions over several weeks, and clearance rates for psoriasis reach 60 to 70%.
How UV Light Affects the Skin
UV light penetrates the outer layers of skin and interacts directly with immune cells there. It suppresses both local and body-wide immune responses by changing the behavior of several cell types, including the keratinocytes that form your skin’s surface, specialized immune-presenting cells called Langerhans cells, and various white blood cells. After UV exposure, the skin produces anti-inflammatory signaling molecules that dial down the immune overreaction driving conditions like psoriasis and eczema.
This immune-calming effect is what makes phototherapy useful for autoimmune and inflammatory skin diseases. The UV light essentially tells your skin’s immune system to stand down in a targeted area, reducing redness, scaling, and itching without the systemic effects of oral medications.
Types of UV Light Therapy
Not all phototherapy is the same. The type your dermatologist recommends depends on your condition, skin type, and how much of your body is affected.
Narrowband UVB
Narrowband UVB (NB-UVB) is the most commonly used form of phototherapy today. It delivers light almost exclusively at 311 nanometers, a very precise wavelength that’s effective against skin inflammation while minimizing unnecessary UV exposure. Because it doesn’t require any medication beforehand, it’s more convenient than older approaches and has become the standard first-line phototherapy for most patients.
Broadband UVB
Broadband UVB uses a wider range of wavelengths, typically 280 to 320 nm for conventional units or 305 to 325 nm for selective versions. It was the original form of UV phototherapy and is still used in some settings, but narrowband UVB has largely replaced it because the focused wavelength tends to be more effective with fewer side effects.
PUVA Therapy
PUVA combines a plant-derived photosensitizing compound called psoralen with UVA light (320 to 400 nm). You take psoralen by mouth or apply it to your skin before the light session. The compound binds to proteins in your skin cells, and when UVA light activates it, it triggers a biological response that slows excessive cell growth and alters immune function. PUVA is typically reserved for cases that don’t respond well to UVB alone, or for specific conditions like vitiligo. The added step of taking a sensitizing medication and the higher long-term risk profile make it less convenient than standalone UVB.
Conditions Treated With Phototherapy
Psoriasis is the most common reason people receive UV light therapy, particularly moderate-to-severe plaque psoriasis that covers too much skin for topical treatments alone. It’s also widely used for atopic dermatitis (eczema), vitiligo, and several less common conditions including cutaneous T-cell lymphoma and certain types of chronic itch.
For psoriasis specifically, NB-UVB phototherapy achieves significant clearance in 60 to 70% of patients who complete a full course of treatment. “Clearance” in this context means a major visible reduction in plaques, though some residual patches may remain. Patients who respond well often enjoy months of remission before needing maintenance sessions or a repeat course.
What a Treatment Course Looks Like
A typical course of NB-UVB phototherapy involves three sessions per week, with at least 24 hours between each session. Most people need 20 to 36 sessions to see significant improvement, which means the full course takes roughly 7 to 12 weeks.
Each appointment lasts about 15 to 30 minutes total, though the actual light exposure is much shorter. Early sessions may last only a few seconds to a few minutes of UV exposure. The dose gradually increases over time based on how your skin responds. Much of the appointment time is spent on preparation and safety checks rather than standing in the light booth itself. After completing a course, your dermatologist will typically schedule follow-up visits every three months for the first year, then every six months after that.
Home-Based vs. Office-Based Treatment
Traveling to a clinic three times a week for months is a significant commitment, which is why home phototherapy units have become increasingly popular. A large randomized trial (the LITE trial) found that home-based narrowband UVB was just as effective as office-based treatment for plaque and guttate psoriasis. In fact, 35.6% of home-therapy patients achieved a 75% reduction in disease severity, compared to 24.1% in the office group. Both approaches were well tolerated, with no patients dropping out due to side effects.
Home units do carry a slightly higher rate of mild sunburn-like reactions (5.9% of treatments vs. 1.2% in the office), likely because clinic staff can more precisely calibrate the dose. If you use a home unit, your dermatologist will provide a dosing schedule and monitor your progress at regular check-ins. Home units require a prescription and typically cost several hundred to a few thousand dollars, though some insurance plans cover them.
Side Effects and Risks
The most common short-term side effect is skin redness, similar to a mild sunburn. This usually fades within a day or two. Other possible reactions include dry skin, itching, blistering (if the dose is too high), and changes in pigmentation. Darker or lighter patches can develop in treated areas, though these are usually temporary.
Long-term, the main concern is skin cancer risk from cumulative UV exposure. The evidence suggests this risk becomes meaningful only after very high numbers of sessions. A systematic review found that caution is warranted beyond 250 sessions of PUVA or 300 sessions of UVB, as the cumulative UV dose at that point may increase the risk of basal cell carcinoma, squamous cell carcinoma, or melanoma. For context, a single course of treatment is 20 to 36 sessions, so reaching those thresholds would take many years and multiple courses. Patients who combine PUVA and UVB over their lifetime face a higher cumulative risk because the two may have a synergistic effect. Accelerated skin aging, similar to what you’d see from years of sun exposure, is also possible with prolonged use.
Eye protection is required during every session. UV exposure to unprotected eyes can damage the cornea and, over time, increase cataract risk. You’ll wear UV-blocking goggles throughout each treatment.
Blue Light Therapy for Newborn Jaundice
A related but distinct form of phototherapy uses blue light (not UV) to treat jaundice in newborns. This is worth mentioning because it often comes up in the same searches, though the mechanism is entirely different from UV phototherapy for skin conditions.
Newborn jaundice happens when a baby’s immature liver can’t process bilirubin fast enough, causing a yellowish tint to the skin. Blue light at around 460 nm is the preferred treatment because bilirubin absorbs blue light more readily than other wavelengths. When the light hits bilirubin molecules in the baby’s skin, it converts them from a form that’s difficult for the body to eliminate into water-soluble versions that can be excreted through bile and urine. This prevents bilirubin from building up to dangerous levels that could harm the brain. Babies’ eyes are covered during treatment as a precaution against potential retinal damage from prolonged blue light exposure.