Photosensitivity: medications and conditions that make UVA dangerous

For most people, UVA is a slow problem — damage measured in decades. For someone who is photosensitive, it is an immediate one. And because the great majority of drug and disease photosensitivity operates in the UVA band rather than the UVB band, the usual sun advice fails these patients in a specific and predictable way: it protects them outdoors at noon and abandons them in the car, at the office window, and in November.

Why photosensitivity is a UVA problem

A photosensitising reaction needs a molecule — a drug, a metabolite, a porphyrin — that absorbs light and passes the energy on. Whether that molecule triggers indoors depends entirely on where its absorption peak sits.

Most photosensitising drugs have extended conjugated ring systems that absorb in the 320–400 nm range. That is squarely UVA. Three consequences follow, and together they explain nearly every confusing feature of these reactions:

• Glass does not help. Windows transmit 50–75% of UVA — see does glass block UVA.
• Cloud does not help much. Overcast still passes 20–40% of UVA, and thin cloud 80% or more.
• Winter and early morning do not help proportionally. UVA falls only about fourfold from midsummer to midwinter, where the sunburning band falls by ten — so the UVA share of what remains actually rises.

It also explains why the UV Index is a poor guide for these patients. A sunburn-weighted number responds to the band that is not causing their reaction. A patient told "the UV Index is only 2, you're fine" can react that afternoon.

Phototoxic vs photoallergic

Phototoxicity is far more common and, in principle, can affect anyone given enough drug and enough light. Photoallergy affects a susceptible minority and is more often associated with topical agents than systemic ones. The subsequent post-inflammatory hyperpigmentation can outlast the reaction itself by months, and is itself worsened by continued UVA exposure.

The main drug classes

This is an orientation list, not a complete one — several hundred agents have been implicated. Frequency of reports varies enormously between drugs within the same class.

Conditions where UVA is the trigger

• Polymorphous light eruption (PMLE) — the most common photodermatosis, affecting a significant share of adults in temperate climates. An itchy papular rash appearing hours to days after the first substantial sun of the season. The action spectrum is predominantly UVA, which is why so many patients report reacting despite diligent SPF use — their sunscreen was blocking the wrong band.
• Solar urticaria — rare; hives within minutes of exposure. Action spectra vary between patients and often extend into UVA and visible light, so individual phototesting is genuinely useful here.
• Lupus erythematosus — photosensitivity is a diagnostic feature. Both subacute cutaneous lupus and discoid lupus flare with UVA as well as UVB, and exposure can trigger systemic as well as cutaneous activity. Rigorous, year-round, broad-spectrum protection is standard management rather than an optional extra.
• The porphyrias — the important special case. Porphyrins absorb most strongly in the Soret band near 400–410 nm, which straddles the boundary between the top of UVA and violet visible light. In erythropoietic protoporphyria this produces burning pain within minutes, often with no visible rash. Because the trigger is partly visible, conventional sunscreens are near-useless: these patients need iron-oxide-tinted or opaque physical blocks, the same visible-light protection that works for melasma, and window filming rated to 400 nm and beyond.
• Chronic actinic dermatitis — persistent eczematous eruption on exposed skin, typically in older men, with broad UVA, UVB and sometimes visible-light sensitivity.
• Xeroderma pigmentosum — inherited failure of DNA repair, requiring near-total avoidance of UV including indoor sources.

Protection that actually works

Standard advice — "SPF 30, avoid midday" — is calibrated for sunburn and therefore for UVB. For a UVA-sensitive patient it is close to useless. What is needed instead:

1. Sunscreen chosen for UVA-PF, not SPF. Look for the EU circled-UVA mark, PA++++, or a five-star Boots rating. Filters with strong, photostable UVA coverage — zinc oxide, bemotrizinol, bisoctrizole, well-stabilised avobenzone — are what matters here. Decoding the labels is the single highest-value thing a photosensitive person can learn.
2. Add visible-light protection if porphyria, solar urticaria or severe melasma is involved. That means iron oxides or an opaque physical block; no clear sunscreen covers 400–450 nm.
3. Film the windows. Car side windows, home and office glazing. Clear films blocking over 99% to 400 nm are widely available and do not need to be dark. This is the intervention that most changes daily life for someone reacting indoors.
4. Clothing over chemistry where possible. UPF-rated fabric, wide brims, gloves for driving. Textiles do not degrade over the day, do not wash off, and do not depend on being reapplied.
5. Consider indoor sources if you are severely photosensitive — bare fluorescent tubes and uncovered halogen fittings emit small amounts of UVA that are irrelevant to most people and not to you. UVA indoors covers what each source emits.
6. Watch UVA, not the UV Index. The UVA Index exists precisely because the standard index goes quiet in the conditions where UVA stays high — overcast, winter, morning and afternoon, behind glass.

Photosensitivity is one of the few situations where the difference between "UV" and "UVA" stops being a technicality and becomes the thing that determines whether your protection works. If you are managing it, find out which band your trigger sits in — and then protect against that one.