Residency · Residency · Dermatology
Drug-Induced Photosensitivity
Introduction
Drug-induced photosensitivity encompasses cutaneous reactions triggered by the interaction between a photosensitizing agent (systemic or topical) and ultraviolet or visible light. These reactions are among the most common adverse drug effects encountered in dermatology and can be classified as phototoxic or photoallergic.
Phototoxicity vs. Photoallergy
Phototoxic Reactions
Phototoxic reactions result from direct tissue damage caused by reactive oxygen species generated when the drug absorbs UV photons, making this a non-immunologic process. They can occur in any individual given sufficient drug concentration and UV dose, as the reaction is dose-dependent. Onset occurs within minutes to hours after UV exposure, and the clinical presentation resembles an exaggerated sunburn with erythema, edema, and vesiculation confined strictly to sun-exposed areas. No prior sensitization is required, so these reactions can occur on first drug exposure. Histology reveals necrotic keratinocytes (sunburn cells), dermal edema, and a sparse inflammatory infiltrate.
Photoallergic Reactions
In photoallergic reactions, UV light converts the drug to a hapten that binds to a carrier protein, triggering a type IV delayed hypersensitivity response. These reactions affect only sensitized individuals, making them idiosyncratic and not dose-dependent. Onset occurs 24 to 72 hours after UV exposure, requiring a prior sensitization period of 1 to 2 weeks. The clinical presentation is an eczematous dermatitis that may extend beyond sun-exposed sites. Histology shows spongiotic dermatitis with a perivascular lymphocytic infiltrate, similar to allergic contact dermatitis.
| Feature | Phototoxic | Photoallergic |
|---|---|---|
| Mechanism | Direct ROS-mediated tissue damage | Type IV delayed hypersensitivity |
| Immune-mediated | No | Yes |
| Dose-dependent | Yes | No |
| Prior sensitization | Not required | Required (1–2 weeks) |
| Onset after UV | Minutes to hours | 24–72 hours |
| Morphology | Exaggerated sunburn | Eczematous dermatitis |
| Distribution | Strictly sun-exposed | May extend beyond sun-exposed |
| Cross-reactivity | No | Yes (structurally related agents) |
| Histology | Sunburn cells, sparse infiltrate | Spongiotic dermatitis |
<image>Side-by-side comparison diagram of phototoxic reaction (exaggerated sunburn pattern with sharp cutoff at clothing lines) versus photoallergic reaction (eczematous eruption extending beyond sun-exposed areas) with mechanistic pathways illustrated</image>
Major Drug Classes Causing Photosensitivity
Antibiotics
Among antibiotics, tetracyclines are the most common cause of drug photosensitivity, with doxycycline being the principal culprit through a phototoxic, dose-related, UVA-mediated mechanism. Fluoroquinolones, particularly lomefloxacin and sparfloxacin, are both phototoxic and potentially photocarcinogenic. Sulfonamides such as trimethoprim-sulfamethoxazole can cause both phototoxic and photoallergic reactions. Voriconazole is phototoxic, and chronic use is associated with accelerated photoaging and squamous cell carcinoma.
Cardiovascular Agents
Hydrochlorothiazide causes both phototoxic and photoallergic reactions and has been associated with an increased risk of lip SCC and BCC with chronic use. Amiodarone is phototoxic and causes blue-gray discoloration in photoexposed areas with prolonged use. Calcium channel blockers such as nifedipine and diltiazem are less common causes.
NSAIDs
Piroxicam is a classic photoallergic agent that cross-reacts with thimerosal due to a shared thiosalicylate moiety. Ketoprofen in topical formulation is a potent photoallergen that cross-reacts with fenofibrate and benzophenone sunscreens. Naproxen is phototoxic and associated with pseudoporphyria.
Psychiatric Medications
Chlorpromazine is phototoxic and causes blue-gray pigmentation with chronic use. Selective serotonin reuptake inhibitors have rare but documented photosensitivity.
Chemotherapeutic and Targeted Agents
5-Fluorouracil causes phototoxic reactions and a UV recall phenomenon. Vemurafenib produces UVA-mediated phototoxicity in nearly 50% of patients and increases SCC risk. Vandetanib can cause severe phototoxic reactions.
Other Important Agents
Psoralens (8-MOP, 5-MOP) produce therapeutic phototoxicity that is exploited in PUVA therapy. St. John's Wort (Hypericum perforatum) contains hypericin, a potent photosensitizer. Retinoids such as isotretinoin reduce stratum corneum thickness, increasing UV penetration.
Pseudoporphyria
Pseudoporphyria is a drug-induced bullous photosensitivity that mimics porphyria cutanea tarda clinically and histologically, but with no abnormality in porphyrin metabolism (normal urinary and serum porphyrins). Common culprits include naproxen (especially in children), furosemide, tetracyclines, and COX-2 inhibitors. It is also seen in chronic renal failure and dialysis patients. Histology shows cell-poor subepidermal blisters similar to PCT but with negative DIF (versus PCT, which shows vessel wall IgG/C3).
<image>Clinical photograph showing tense vesicles and bullae on the dorsal hands with skin fragility and milia formation in a patient with naproxen-induced pseudoporphyria</image>
Diagnostic Approach
History
The diagnostic approach begins with establishing the temporal relationship between drug initiation and photosensitivity onset. Most drug photosensitivity is UVA-mediated, meaning patients may react through window glass, and the action spectrum provides important clues. All medications must be reviewed, including over-the-counter products, herbal supplements, and topical agents.
Investigation
Phototesting with MED determination to UVA and UVB will show reduced MED in phototoxicity. Photopatch testing, in which standard allergens are applied in duplicate with one set irradiated with UVA, is the gold standard for diagnosing photoallergy. Porphyrin studies are essential to exclude true porphyria in bullous photosensitivity. Drug rechallenge with UV is rarely needed but can confirm causality.
Differential Diagnosis
The differential diagnosis includes polymorphous light eruption, solar urticaria, lupus erythematosus, porphyria cutanea tarda, dermatomyositis, and pellagra.
Management
Drug withdrawal is the primary intervention. Phototoxic reactions resolve with drug discontinuation, while photoallergic reactions may persist as chronic actinic dermatitis. Photoprotection with broad-spectrum sunscreen emphasizing UVA coverage (high PPD/PA rating), protective clothing, and window films is essential. Topical corticosteroids provide symptomatic relief of the acute eruption. Cool compresses and emollients help manage phototoxic burns. Systemic corticosteroids may be necessary for severe photoallergic reactions. The reaction should be documented and patients counseled on cross-reactive agents.
<image>Flowchart for the diagnostic workup of suspected drug-induced photosensitivity including timeline assessment, phototesting, photopatch testing, porphyrin studies, and management algorithm</image>
Special Considerations
Voriconazole Phototoxicity
Chronic voriconazole use (beyond 6 months) is associated with photoaging, actinic keratoses, and aggressive SCCs. The mechanism involves generation of retinyl esters and fluorescent metabolites. Transplant patients on voriconazole require aggressive photoprotection and skin cancer surveillance, and switching to alternative antifungals should be considered when possible.
Hydrochlorothiazide and Skin Cancer
Both the European Medicines Agency and FDA have acknowledged the association between hydrochlorothiazide and skin cancer. The odds ratio is 4 to 7 for lip SCC with cumulative high-dose exposure. Risk increases with cumulative dose, and alternative antihypertensives should be considered in high-risk patients.
Key Clinical Pearls
Most drug photosensitivity is UVA-mediated, meaning patients may react through window glass and standard UVB sunscreens may be insufficient. Phototoxicity is dose-dependent and can occur on first exposure, while photoallergy is idiosyncratic and requires prior sensitization. Pseudoporphyria mimics PCT but porphyrin levels are normal, so porphyrins should always be checked in any bullous photodermatosis. Voriconazole photosensitivity can drive aggressive squamous cell carcinomas, especially in immunosuppressed patients. Piroxicam photoallergy cross-reacts with thimerosal, and ketoprofen photoallergy cross-reacts with benzophenone sunscreens.
References
- Drucker AM, Rosen CF. Drug-induced photosensitivity: culprit drugs, management and prevention. Drug Saf. 2011;34(10):821-837.
- Blakely KM, Drucker AM, Rosen CF. Drug-induced sun sensitivity — an update. Drug Saf. 2019;42(7):827-847.
- Gelot P, Dutartre H, Khammari A, et al. Vemurafenib skin phototoxicity: clinical features and modulating factors. J Eur Acad Dermatol Venereol. 2020;34(10):2272-2279.
- Cowen EW. Drug-induced photosensitivity. In: Kang S, et al., eds. Fitzpatrick's Dermatology. 9th ed. McGraw-Hill; 2019.
