Yes, phototherapy does make you tan. The ultraviolet light used in medical phototherapy stimulates the same pigment-producing cells that darken your skin in sunlight. Most people notice their skin getting progressively darker over the course of treatment, and this tanning is one of the most common side effects reported by patients.
Why Phototherapy Causes Tanning
Medical phototherapy, most commonly narrowband UVB (NB-UVB), uses ultraviolet light in the 290 to 320 nanometer range. These are the same wavelengths that trigger melanin production in your skin when you’re out in the sun. Your skin’s pigment cells respond to the UV exposure by producing more melanin as a protective measure, which is exactly what a tan is.
The key difference between phototherapy and a tanning bed is purpose and precision. Medical phototherapy delivers a controlled dose of a narrow band of UV light to treat conditions like psoriasis, eczema, or vitiligo. Tanning beds emit a much broader spectrum, with most of their output in the UVA range (commercial beds typically contain only 0.8% to 4.2% UVB). But because phototherapy relies heavily on UVB wavelengths, it’s quite effective at triggering melanin production, even though that’s not the goal.
How the Tan Develops During Treatment
Tanning from phototherapy isn’t instant. It builds gradually over the course of your sessions. A Korean study tracking narrowband UVB patients with psoriasis found that skin darkened continuously over the first five weeks of treatment, then hit a plateau where it didn’t get much darker even with continued sessions. This mirrors what happens with sun exposure: your skin reaches a maximum tanning capacity based on your genetics and levels off.
The darkening tends to be more noticeable in areas directly exposed to the light. If you’re getting full-body treatment in a booth, most of your skin will tan. If you’re getting targeted treatment for a specific patch of skin, only that area and surrounding skin will darken, which can create an uneven appearance.
Skin Type Affects How Much You Tan
How dark you get depends heavily on your natural skin type. The Fitzpatrick scale, originally developed specifically to guide UV dosing for phototherapy, classifies skin into six types based on tanning and burning tendencies:
- Type I: Always burns, never tans
- Type II: Usually burns, tans minimally
- Type III: Occasionally burns, usually tans evenly
- Type IV: Rarely burns, always tans well
- Type V: Very rarely burns, tans very easily
- Type VI: Never burns, always tans
If you’re a Type I or II, you’ll likely burn before you tan noticeably, which is why your dermatologist starts with very low doses and increases gradually. If you’re a Type IV or higher, you can expect significant darkening. Your treatment dose is calibrated around these differences, but tanning to some degree is nearly unavoidable for anyone whose skin is capable of producing melanin.
Dark Patches and Uneven Color
General, all-over tanning is one thing. But phototherapy can also cause localized dark patches that are more noticeable and sometimes more bothersome than the overall tan. This is especially common around areas where you had active skin disease.
When psoriasis plaques or eczema patches heal, they often leave behind a darkened area called postinflammatory hyperpigmentation. The UV from phototherapy can make these already-dark spots even darker. For many patients, this is an acceptable trade-off for clearing their skin condition, but it can be cosmetically frustrating, particularly on visible areas like the arms or face.
One practical workaround: once a plaque has fully cleared and only the dark mark remains, you can apply sunscreen to that specific spot before your phototherapy session. This shields the healed area from additional UV while still letting the light reach any remaining active disease. A topical retinoid applied to those dark patches (outside of treatment sessions) can also help the hyperpigmentation fade faster.
PUVA vs. NB-UVB Tanning
If your treatment involves PUVA (psoralen plus UVA light) rather than narrowband UVB, the tanning dynamics are somewhat different. PUVA uses a photosensitizing medication that makes your skin more reactive to UVA light, which can produce intense, sometimes uneven pigmentation. NB-UVB tends to produce more cosmetically even repigmentation, which is one reason it has become the preferred phototherapy option for conditions like vitiligo. The tan from NB-UVB, while still noticeable, generally looks more uniform than what PUVA produces.
Protecting Areas You Don’t Want to Tan
During full-body phototherapy, certain areas are routinely shielded. Men typically wear a genital shield, and many clinics recommend protecting the face and neck if those areas don’t need treatment, since facial skin is thinner and more prone to UV damage and visible tanning.
For targeted conditions like vitiligo on the lips, clinicians sometimes use simple cardboard cutouts or face masks with a window exposing only the treatment area. This low-tech approach is surprisingly effective since cardboard blocks UV light well. If you’re concerned about tanning in specific areas, ask your phototherapy team about shielding options before your first session. Most clinics are happy to accommodate these requests.
How Long the Tan Lasts
The tan from phototherapy is not permanent, but it takes longer to fade than you might expect. Research tracking patients after their final NB-UVB session found that skin color began slowly recovering once treatment stopped, but the process was gradual. It took about 10 weeks for any significant lightening to become apparent.
Full recovery to your original skin color took considerably longer. In the same study, patients’ skin measurements didn’t return to their pre-treatment baseline until roughly 8 to 10 months after completing phototherapy. So if you finish a course of treatment in January, you may not be back to your natural color until fall. This timeline can vary based on your skin type, how many sessions you had, and how much cumulative UV your skin absorbed.
The gradual nature of this fading also means you won’t notice dramatic changes week to week. It’s a slow, steady return to baseline rather than a sudden shift, much like how a deep summer tan takes months to fully disappear over winter.