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Purpose and principlesIndications, selection and cautionsAssessment before treatmentTreatment approachRegimens, contraindications and interactionsComplications, monitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Phototherapy safety

Dermatology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

RapidFull
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Severe phototoxic burn or collapse

Painful confluent erythema, extensive blistering, skin loss, eye pain or visual change, syncope, heat illness or systemic symptoms after a session exceeds an expected mild treatment reaction and may require burn, ophthalmic or acute medical care.

Action: Stop further exposure, cool and assess ABCDE, surface area, hydration and eye symptoms, contact the phototherapy service immediately and arrange emergency or specialist burn assessment when severe; record lamp, delivered dose, positioning, medicines and timing.

Purpose and principles

Select supervised narrowband UVB or PUVA appropriately, establish an individual starting dose, prevent burns and ocular or genital exposure, reconcile photosensitising medicines, and monitor cumulative ultraviolet harm and treatment benefit.

Narrowband UVB concentrates wavelengths around the therapeutically useful UVB band and alters keratinocyte proliferation, antigen presentation and T-cell signalling. UVA penetrates deeper but has limited therapeutic effect without a photosensitiser in PUVA. Psoralen intercalates with DNA and, after UVA activation, forms photoadducts that reduce proliferation and immune activity. The same mechanisms damage normal tissue, explaining delayed erythema, photoageing, pigment change, cataract risk without protection and cumulative carcinogenesis.

Suitability is disease and person specific. Confirm diagnosis, extent, previous treatment, likely ability to attend two or three times weekly and whether targeted local treatment or systemic therapy is preferable. Review melanoma and keratinocyte-cancer history, radiotherapy and arsenic exposure, lupus and other photosensitivity, inherited DNA-repair disease, immunosuppression, pregnancy plans, cataract risk and every prescribed or non-prescribed medicine. Narrowband UVB can be used in pregnancy when clinically appropriate; oral PUVA and its psoralen exposure require a separate reproductive and ocular decision.

Indications, selection and cautions

Expected mild erythema

A faint warm response without significant pain may occur after a planned increment and resolves within the unit's accepted interval.

Phototoxic burnRed flag

Confluent tender erythema, oedema, blistering or pain appearing hours to three days after treatment indicates excessive ultraviolet exposure.

Polymorphic light eruption

A new itchy papular eruption in exposed treatment areas can arise during a course and should be shown to staff before the next dose.

Patchy exposure error

Geometric erythema or pigmentation follows changed clothing, jewellery, stance, topical product or shielding and helps reconstruct uneven ultraviolet delivery.

PUVA ocular concern

Eye pain, photophobia or visual change after psoralen with absent or inadequate UVA-protective eyewear requires urgent ophthalmic assessment.

Cumulative photodamage

Lentigines, telangiectasia, coarse wrinkling and multiple keratotic lesions reflect long-term ultraviolet injury and alter further-course decisions.

Suspicious skin lesion

A new changing pigmented lesion, non-healing ulcer or enlarging keratotic nodule during surveillance needs diagnostic assessment outside the phototherapy schedule.

Red flags requiring action

  • Blistering, severe or persistent pain, extensive erythema, mucosal or eye symptoms, fever, collapse, suspected machine or dosing error, a new photosensitising medicine, a changing pigmented lesion, or treatment in a person with major skin-cancer or photosensitivity risk requires prompt withholding and specialist review.

Assessment before treatment

  1. 01
    Pre-treatment skin and risk assessmentFirst step
    Why
    Confirm indication and identify photosensitivity, malignancy and practical contraindications.
    Interpretation and limitations
    Document skin phenotype without assuming ethnicity predicts burn response, full skin examination, cancer history, photosensitive disease, immune status, pregnancy and ability to follow protection instructions.
  2. 02
    Medicine and product reconciliation
    Why
    Find systemic and topical photosensitisers before dosing.
    Interpretation and limitations
    Record every new antibiotic, diuretic, retinoid, NSAID, herbal product, perfume, tar and topical medicine and ask the unit to assess significance before exposure.
  3. 03
    Minimal erythema dose
    Why
    Estimate individual UVB sensitivity and a safe therapeutic starting dose.
    Interpretation and limitations
    Expose small sites to graded UVB and read around 24 hours under the unit protocol; changed medicines or biology can make an old result unreliable.
  4. 04
    Minimal phototoxic dose
    Why
    Estimate UVA response after a standard psoralen exposure for PUVA planning.
    Interpretation and limitations
    Give psoralen at the protocol interval, expose test sites and read the delayed response around three to four days; treatment does not start before the result is incorporated.

Treatment approach

01First-line course planningSelect wavelength and establish baselineFirst stepFirst lineTopical treatment is insufficient and supervised phototherapy is an appropriate next option.+
  1. 1Confirm diagnosis, treatment goal, extent, prior therapies, skin-cancer and photosensitivity risk, medicines, pregnancy context and attendance feasibility.
  2. 2Choose narrowband UVB, PUVA or another unit modality for the disease and obtain consent covering burns, eye and genital protection, photoageing, cancer risk and alternatives.
  3. 3Perform MED, MPD or an approved phenotype-based start, document shielding and provide written pre-session product and medication instructions.
02Session safetyCheck before every incrementEscalationThe patient attends for an escalating phototherapy dose.+

Regimens, contraindications and interactions

Photosensitises skin so calibrated UVA can treat selected psoriasis, eczema, cutaneous lymphoma, vitiligo or other specialist indications.

Oral psoralen for PUVA

Take the hospital-pharmacy supplied number of tablets based on height and weight at the exact unit-specified interval, usually 2 hours before UVA; never repeat a vomited or missed dose without phototherapy-unit instruction.

Causes nausea, prolonged UVA sensitivity and ocular risk. Wear tested UVA-protective glasses for at least 12 hours, protect from daylight until day's end, review pregnancy and liver factors and avoid unsupervised additional UVA.

Improves comfort and scale without deliberately altering the ultraviolet prescription.

Bland water-based emollient

Use regularly between sessions for dryness; on treatment days apply only the unit-approved non-SPF water-based product at the permitted interval, commonly at least 2 hours before or immediately after exposure.

Oily, perfumed or unapproved products can block or intensify exposure and produce burns or patchy pigmentation. All emollient residue on fabrics increases fire severity.

Reduces inflammation from a limited phototoxic reaction alongside cooling, emollient and analgesia.

Site-appropriate topical corticosteroid after a minor burn

Use only when advised by the phototherapy service, applying the specified potency thinly to symptomatic inflamed skin for the defined short course while ultraviolet treatment is withheld.

Blistering, extensive pain, eye symptoms or systemic illness needs urgent assessment rather than home steroid alone;

Complications, monitoring and follow-up

  • At each visit record delayed erythema grade and duration, pain, pruritus, missed sessions, intercurrent illness, new medicines and unintended sunlight.
  • Log wavelength, cabinet, delivered dose, cumulative dose, body position and shields so a reaction can be reconstructed precisely.
  • Measure disease-specific response at planned intervals and stop for inadequate benefit, repeated burning, impractical attendance or risk that now outweighs benefit.
  • Review the entire medicine and topical-product list whenever another prescriber starts treatment, especially antibiotics, retinoids or photosensitising agents.
  • For PUVA confirm psoralen timing, nausea, eyewear and daylight protection on each treatment day and arrange risk-based eye review.
  • Maintain a lifetime exposure and skin-cancer risk record and provide self-examination and rapid lesion-change advice after high cumulative treatment.

Special situations

The burn is delayed

A normal appearance on leaving the cabinet cannot certify safe dosing because UVB erythema and PUVA phototoxicity evolve later.

Clothing becomes dosimetry

A newly exposed strip of untanned skin receives the current high course dose without prior hardening and can burn geometrically.

UVA crosses window glass

Oral psoralen eye and skin precautions apply indoors near bright windows as well as outside, unlike advice based only on direct sun.

Skin tone is not a dose

Visible erythema can be subtle in deeply pigmented skin and phenotype does not replace MED testing, symptoms and palpation.

Home treatment remains prescribed

A home cabinet requires a calibrated device, treatment schedule, training, locked access and specialist outcome review rather than patient-led escalation.

Clearance is not cure

Phototherapy can induce remission but relapse and cumulative toxicity mean routine indefinite maintenance is rarely the safe default.

Common pitfalls

  1. 01

    Recommending a tanning salon as equivalent to narrowband UVB.

  2. 02

    Escalating today's dose without asking about erythema that peaked two days after the previous session.

  3. 03

    Ignoring a new antibiotic, retinoid, perfume or herbal product during a course.

  4. 04

    Changing underwear, jewellery or hairstyle after the covered skin has missed earlier dose increments.

  5. 05

    Allowing oral PUVA without tested UVA-protective eyewear and near-window advice.

  6. 06

    Using redness alone to assess a burn in richly pigmented skin and overlooking pain, heat, oedema or blistering.

  7. 07

    Continuing routine maintenance despite poor response, repeated burns or high cancer risk.

  8. 08

    Failing to record cumulative exposure across different hospitals or home courses.

Practice

Two practice questions

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Question 1 of 2
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DermatologyOriginal SBA

New photosensitiser before treatment

A patient midway through an escalating narrowband-UVB course starts doxycycline and attends for the next session without erythema. What is the safest immediate action?

Choose one answer, then submit
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Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • BAD narrowband UVB phototherapyMED testing, session process, protection, burns, medicines and cumulative risk.
  • BAD oral PUVA phototherapyPsoralen timing, MPD testing, eyewear, daylight protection, adverse effects and long-term risk.
  • NICE psoriasis recommendationsNarrowband-UVB indications, frequency, response, relapse and treatment-sequencing principles.
  • NHS sunburnRecognition and first aid for minor ultraviolet injury and urgent assessment thresholds.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom
Consistency is a safety control. The same body position, shielding and clothing boundary must be maintained as dose rises. Perfume, deodorant, tar products and oily topical preparations can alter absorption or provoke patchy phototoxicity. A water-based non-SPF emollient may be permitted at a unit-defined interval, but only the treating service should decide. The unit documents machine calibration, individual dose and cumulative course. Home phototherapy requires the same prescription, device governance, training and review and is not self-directed sun exposure.

Key points

  • Phototherapy is calibrated medical ultraviolet exposure delivered by a trained unit; natural sunlight and commercial tanning beds cannot substitute for its wavelength, dosimetry or safety checks.
  • Narrowband UVB reduces cutaneous inflammation and is used for psoriasis, eczema, vitiligo and selected photodermatoses after diagnosis-specific assessment.
  • PUVA combines psoralen photosensitisation with UVA, penetrates more deeply and carries more nausea, ocular precautions, phototoxicity and cumulative skin-cancer burden.
  • A minimal erythema dose can establish UVB sensitivity, while a minimal phototoxic dose after psoralen helps select oral PUVA exposure; skin-type protocols are alternatives when testing is unsuitable.
  • Ask before every session about erythema, itch, illness, missed visits, new medicines, topical products, sun exposure and any change in clothing or jewellery coverage.
  • Eye protection must fit and remain in place during exposure; male genital shielding is used unless the prescribed protocol deliberately treats that site.
  • Oral psoralen is usually taken two hours before UVA; UVA-protective glasses are worn for at least 12 hours, including near windows, with longer advice for children or cataract risk.
  • Do not expose previously covered untanned skin to an escalated dose by changing underwear, hairstyle, jewellery or position part-way through a course.
  • A burn can peak one to three days after treatment, so dose escalation uses the previous delayed reaction rather than appearance immediately after leaving the cabinet.
  • Record cumulative sessions and ultraviolet dose, response, burns and skin-cancer history and stop a burdensome or ineffective course instead of converting it into routine maintenance.
05
Pre-session erythema check
Why
Prevent escalation on top of an evolving delayed burn.
Interpretation and limitations
Ask about onset, pain and duration and inspect treated skin; symptomatic or persistent erythema leads to withholding or dose reduction under the protocol.
  • 06
    Cumulative-dose and response record
    Why
    Balance therapeutic benefit against burns and long-term ultraviolet burden.
    Interpretation and limitations
    Record wavelength, joules or millijoules delivered, number of sessions, shielded sites, interruptions, reactions and a reproducible disease outcome.
  • 07
    Skin-cancer surveillance
    Why
    Detect malignancy in a person with substantial PUVA, UVB or other carcinogenic exposure.
    Interpretation and limitations
    Use risk-stratified full-skin examination and biopsy suspicious change; a cumulative number alone does not replace inspection or previous-radiotherapy history.
    • 1
    Ask about delayed erythema, pain, itch, illness, new medicines, sun exposure, missed sessions and changed clothing, hairstyle, jewellery or topical product.
  • 2Inspect when symptoms or protocol require it, fit goggles and genital or facial shielding and reproduce body position and coverage.
  • 3Deliver and record the protocol-calculated dose, withholding or reducing after burns, interruption, photosensitiser change or machine concern.
    • 03Oral PUVA dayProtect photosensitised skin and eyesOral psoralen plus UVA has been selected by a specialist unit.+
    1. 1Take the pharmacy-supplied height- and weight-based psoralen dose at the unit-specified time, usually two hours before UVA, and do not replace a missed dose independently.
    2. 2Wear the tested UVA-blocking glasses for at least 12 hours after psoralen, including indoors near windows, and follow 24-hour advice when the unit identifies higher cataract risk.
    3. 3Avoid daylight exposure until the end of daylight that day, protect skin, limit caffeine if symptomatic and report nausea or eye symptoms before another treatment.
    04Burn responseWithhold, assess and reconstruct the dosePainful erythema, oedema or blistering develops after a session.+
    1. 1Stop further phototherapy and contact the unit; assess surface area, blistering, hydration, infection, eye symptoms and need for emergency or burn care.
    2. 2Provide cool compresses, bland emollient and prescribed anti-inflammatory or analgesic care for a minor reaction, avoiding additional sun exposure.
    3. 3Review machine, delivered dose, MED or MPD, medicines, products, shielding and positioning before a senior clinician authorises a reduced restart.
    do not
    apply before sessions unless the unit explicitly permits it.