Light therapy is one of the most effective treatments for psoriasis, with about 70% of patients achieving at least 75% skin clearance after a full course. It works across all skin tones, can be done at home or in a clinic, and remains a frontline option for moderate-to-severe plaque psoriasis. The specific results depend on which type of light therapy you use and how consistently you stick with it.
How Light Therapy Works on Psoriasis
Psoriasis is driven by an overactive immune response. Certain immune cells flood the skin with inflammatory signals, triggering the rapid buildup of skin cells that forms plaques. UV light disrupts this cycle in two ways: it dials down the inflammatory signals fueling plaque growth, and it shifts the immune response toward a calmer, anti-inflammatory state.
This isn’t a vague suppression of the whole immune system. Studies using gene expression profiling show that UV light specifically inhibits the inflammatory pathways responsible for psoriasis while boosting counter-regulatory signals that keep those pathways in check. Importantly, this only happens in plaques that actually respond to treatment, which explains why some stubborn patches may not improve even when surrounding skin clears.
Clearance Rates by Type
Narrowband UVB
Narrowband UVB (NB-UVB) is the most commonly prescribed form of phototherapy for psoriasis. A meta-analysis of over 1,300 patients found that 70.5% achieved PASI 75, meaning their skin improved by at least 75%. Treatment schedules in the studies ranged from twice a week for eight weeks to three times a week for twelve weeks. Some people notice improvement after just a few sessions, with significantly clearer skin possible in about six weeks.
PUVA
PUVA combines a light-sensitizing medication (psoralen) with UVA light. It’s slightly more effective than narrowband UVB: about 80% of patients reach PASI 75, and 44% reach PASI 90 (near-complete clearance). In a large retrospective study, 62% of patients needed only a single treatment course, and roughly two-thirds of those patients maintained their results long-term without needing biologic drugs or additional intensive therapy. The tradeoff is that PUVA carries a higher side effect profile, which is why narrowband UVB is typically tried first.
Excimer Laser
For people with psoriasis limited to a few stubborn patches, the 308-nm excimer laser targets individual plaques with concentrated UVB light. In a study of 120 patients, 66% were at least 90% clear after 10 sessions, and 85% reached that level by 13 sessions. The average treatment course took about seven weeks. Because the laser only hits affected skin, it spares surrounding healthy tissue from UV exposure entirely.
How Long Remission Lasts
After a successful course of light therapy, psoriasis doesn’t come back the next week. A retrospective study comparing remission duration found that patients treated with PUVA stayed clear for an average of about 13 months, while those treated with narrowband UVB averaged about 10 months. The difference wasn’t statistically significant, but PUVA trended toward longer remission by roughly three months. Individual results vary widely: some patients stay clear for well over a year, others relapse sooner. When psoriasis does return, another course of phototherapy is a standard option.
Home Units vs. Clinic Visits
One of the biggest barriers to phototherapy is the time commitment. Going to a clinic two or three times a week for months is genuinely difficult for most people, and missed sessions reduce effectiveness. A large randomized trial of 783 patients, called the LITE trial, directly compared home-based narrowband UVB units to office-based treatment and found that home therapy was just as effective across all skin tones. In fact, home-treated patients trended toward slightly better outcomes, likely because they could stick with the schedule more easily without the burden of travel and time off work.
The benefits were especially strong for patients with darker skin tones, who showed particularly robust improvement with home units. If your dermatologist recommends phototherapy but the logistics of clinic visits feel unworkable, a home unit is a well-supported alternative.
Side Effects and Risks
Short-term side effects are common but generally mild. In an observational study of over 1,200 treatment courses, 19% of patients experienced at least one side effect. The most frequent were redness (8.8%), darkening of treated skin (4.5%), itching with redness (1.9%), and UV burns (1.4%). These are typically manageable and don’t require stopping treatment.
The longer-term concern is cumulative UV exposure and skin cancer risk. One retrospective study tracked 100 patients who underwent phototherapy over five years, completing about 160 total sessions. Of those, 34 developed at least one non-melanoma skin cancer (basal cell or squamous cell carcinoma), and 10 of the 34 developed two. This risk is more relevant with PUVA than with narrowband UVB, and it increases with total lifetime UV dose. For patients who need repeated courses over many years, dermatologists weigh this cumulative exposure when deciding whether to continue phototherapy or switch to other treatments like biologics.
What a Typical Course Looks Like
Most treatment protocols start with two to three sessions per week. Each session is short, often under 20 minutes, with the UV dose gradually increased based on how your skin responds. A full course typically runs 8 to 12 weeks. Your dermatologist will assess your progress along the way and adjust the dose if you’re burning or not responding.
Results aren’t all-or-nothing. Some patients achieve near-complete clearance, others get meaningful but partial improvement. About 15% of patients in the excimer laser studies, for example, saw less than 50% improvement. Thicker, older plaques and certain body areas (elbows, knees, scalp) tend to be more resistant. If phototherapy alone isn’t enough, it can be combined with topical treatments to improve results.