Photosensitivity.
Several sulfa drugs cause photosensitivity — sun-exposed skin reacts to ultraviolet light more strongly than usual, producing redness, burning, or rash on areas left uncovered. Sulfa antibiotics (notably TMP-SMX) and hydrochlorothiazide are among the recognised offenders. Routine sun precautions reduce the risk.
- Common sulfa offenders
- TMP-SMX, other sulfa antibiotics, HCTZ (notable), furosemide, sulfonylureas, sulfasalazine.
- Mechanism
- Drug or metabolite absorbs UV (mostly UVA), forming reactive species that damage skin.
- Two patterns
- Phototoxic (looks like exaggerated sunburn, dose-dependent) and photoallergic (eczema-like, immune-mediated).
- Practical measures
- Protective clothing, broad-spectrum sunscreen, avoiding peak UV, hats and sunglasses.
What it is
Drug-induced photosensitivity describes an exaggerated skin response to ultraviolet light during exposure to a particular drug. The drug or its metabolites absorb UV — usually UVA in the 320–400 nm range — and the resulting energy or reactive intermediates damage skin or trigger immune responses.
Two patterns are recognised. Phototoxic reactions look like an exaggerated sunburn: redness, swelling, and pain on sun-exposed areas, sometimes with blistering. They are dose-dependent (more drug, more sun, more reaction), can occur on first exposure, and resolve when the drug or sun is removed. Photoallergic reactions are immune-mediated, eczema-like (itchy, red, scaly), often appearing days into exposure, and may extend to skin not directly sun-exposed. Sulfa drugs can cause either, more often the phototoxic kind.
Which sulfa drugs
Sulfa antibiotics. Particularly sulfamethoxazole/trimethoprim. Photosensitivity is a recognised effect during and shortly after a course.
Hydrochlorothiazide. One of the more notable photosensitising drugs in routine prescribing. Long-term thiazide use has been associated in some pharmacoepidemiology studies with an increased risk of non-melanoma skin cancer (squamous and basal cell carcinoma), thought to relate to chronic photosensitivity over years; the absolute increase is small.
Other diuretics. Furosemide and other thiazides can also cause photosensitivity, generally less commonly than HCTZ.
Sulfonylureas. Photosensitivity is reported, less common than with thiazides.
Sulfasalazine. Reported.
Topical sulfa. Sulfacetamide and silver sulfadiazine can cause photosensitive reactions on treated skin.
What to expect
The reaction usually appears on areas of the skin actually exposed to the sun: face, neck, V of the chest, backs of hands, forearms, dorsa of the feet. Areas under clothing are spared. The pattern alone often suggests the diagnosis. Duration varies: phototoxic reactions usually settle days after the drug or sun is removed; photoallergic patterns may persist longer and occasionally outlast drug exposure.
Severity ranges from mild redness to painful blistering and skin breakdown in extreme cases. Most patients describe an exaggerated sunburn rather than a frank rash.
What to do
For patients on a sulfa drug — particularly HCTZ long-term — sensible sun-protective measures reduce the risk:
Cover sun-exposed skin with clothing where possible. Use a broad-spectrum sunscreen (one that blocks UVA as well as UVB) at SPF 30 or higher, applied thickly and re-applied as required. Wear a wide-brimmed hat and sunglasses. Avoid peak UV exposure (typically late morning to mid-afternoon). Be aware that UV can pass through clouds and through some glass.
If a photosensitive reaction develops, the first step is to reduce sun exposure. Topical treatments (cool compresses, mild emollients, low-potency topical corticosteroids for inflamed areas) and oral antihistamines for itch are used as needed. For a clear, recurring photosensitivity attributable to a particular drug, a clinician may consider an alternative drug if one is appropriate. The decision belongs to the prescriber.
Severe reactions and red flags
Most photosensitive reactions are uncomfortable rather than dangerous. A few patterns warrant urgent attention:
Severe blistering or skin breakdown is unusual for routine drug photosensitivity and may suggest a different process. Pain (rather than itch) on the skin should prompt assessment, as it can be a feature of more serious reactions. Mucosal involvement is not consistent with simple photosensitivity and warrants attention. More on severity.
Other photosensitising drug classes (for context)
Sulfa drugs are far from the only photosensitising medications. Tetracyclines (especially doxycycline), fluoroquinolones, amiodarone, voriconazole, certain NSAIDs (notably some oxicams), psoralens, and many others can cause photosensitivity. The clinical principle is the same: identify the drug, protect the skin.