Photosensitivity Reactions

ByJulia Benedetti, MD, Harvard Medical School
Reviewed/Revised Oct 2023 | Modified Nov 2023

Photosensitivity, sometimes referred to as a sun allergy, is an immune system reaction that is triggered by sunlight.

  • Sunlight can trigger immune system reactions.

  • People develop itchy eruptions or areas of redness and inflammation on patches of sun-exposed skin.

  • The diagnosis is usually based on a doctor's evaluation.

  • These reactions typically resolve without treatment.

(See also Overview of Sunlight and Skin Damage.)

Photosensitivity reactions include solar urticaria, chemical photosensitization, and polymorphous light eruption and are usually characterized by an itchy eruption on patches of sun-exposed skin. People may inherit a tendency to develop these reactions. Certain diseases, such as systemic lupus erythematosus and some porphyrias, also may cause more serious skin reactions to sunlight.

Solar urticaria

Hives (large, itchy red bumps or welts) that develop after only a few minutes of exposure to sunlight are called solar urticaria. The hives typically last for minutes or hours. A person can be prone to developing solar urticaria for a very long time, sometimes indefinitely. People with large affected areas sometimes have headaches and wheezing and feel dizzy, weak, and nauseated.

Solar Urticaria
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This photo shows solar urticaria in a woman who had been wearing a tank top. These hives appear within minutes of sun exposure.
© Springer Science+Business Media

Chemical photosensitivity

Over 100 substances, swallowed or applied to the skin, are known to cause sun-induced reactions on the skin. A limited number cause most reactions (see table Some Medications and Substances That Sensitize the Skin to Sunlight). There are 2 types of chemical photosensitivity: phototoxicity and photoallergy.

In phototoxicity,

In photoallergy, an allergic reaction causes redness, scaling, itching, and sometimes blisters and spots that resemble hives. This type of reaction can be caused by aftershave lotions, sunscreens, and sulfonamides. Substances that cause photoallergy are capable of doing so only after the person has been exposed to both the substance and sunlight (because sunlight is what makes the substance capable of triggering photoallergy). Photoallergic reactions can also affect areas of skin that have not been exposed to the sun. They usually develop 24 to 72 hours after sun exposure.

Some Medications and Substances That Sensitize the Skin to Sunlight

  1. Antianxiety medications

  2. Antibiotics

    1. Quinolones

    2. Sulfonamides

  3. Antidepressants

    1. Tricyclic antidepressants

  4. Antifungal medications

  5. Antihyperglycemics

    1. Sulfonylureas

  6. Antimalarial medications

  7. Antipsychotics

    1. Phenothiazines

  8. Chemotherapy medications

  9. Diuretics

    1. Thiazides

  10. medications used to treat acne

  11. Heart medications

    1. Pain-relief medications (analgesics)

  12. Sensitizing substances

    1. Fragrances

    2. Furocoumarins present in plants, such as limes, celery, and parsley

    3. Sunscreens

Polymorphous light eruption

This eruption is a reaction to sunlight (primarily UVA light) that is not fully understood. It is one of the most common sun-related skin problems and is most common among women and among people from northern climates who are not regularly exposed to the sun.

The eruption appears as multiple red bumps and irregular, red, raised areas (called plaques) and, rarely, as blisters on sun-exposed skin. These plaques, which are itchy, typically appear 30 minutes to several hours after sun exposure. However, new patches may develop many hours or several days later.

The eruption usually goes away within several days to weeks. Typically, people with this condition who continue to go out in the sun gradually become less sensitive to the effects of sunlight (a process known as hardening).

Polymorphous Light Eruption
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This photo shows red bumps and raised areas on the upper back.
Image courtesy of E. Laurie Tolman, MD.

Diagnosis of Photosensitivity Reactions

  • A doctor's evaluation

  • Sometimes phototesting (skin patch and reaction reproduction test)

There are no specific tests for photosensitivity reactions. A doctor suspects a photosensitivity reaction when a rash appears only in areas exposed to sunlight. A close review of the person's medical history, skin symptoms, any diseases, drugs taken by mouth, or substances applied to the skin (such as medications or cosmetics) may help a doctor pinpoint the type and cause of the photosensitivity reaction. Doctors may do tests to rule out diseases that are known to make some people susceptible to such reactions (such as systemic lupus erythematosus).

When a rash occurs on an area of skin that has been exposed to the sun and the diagnosis is not clear, doctors may do skin patch tests and reaction reproduction tests that involve exposure to UV light (phototesting) when the person is not using any substance that cause photosensitivity reactions. These tests may help clarify which type of photosensitivity reaction may be the cause.

Treatment of Photosensitivity Reactions

  • For photosensitivity, stopping medications or chemicals and sometimes taking corticosteroids

  • For solar urticaria, antihistamines, corticosteroids, or sunscreens or sometimes ultraviolet (UV) light therapy

If possible, any medications or chemicals that could cause photosensitivity should be discontinued after consulting with a doctor. To treat chemical photosensitivity reactions, corticosteroids are applied to the skin and the substance that is causing the reaction is avoided.

People with polymorphous light eruption or photosensitivity caused by systemic lupus erythematosusPhototherapy).

Taking Polypodium leucotomos (a dietary supplement made from certain tropical ferns) or nicotinamide (a form of vitamin B3) by mouth may be helpful in preventing symptoms in people susceptible to photosensitivity caused by sun exposure.

Prevention of Photosensitivity Reactions

All people should avoid excessive sun exposure, but people who are sensitive to sunlight due to any cause should be especially careful and wear protective clothes, avoid sunlight as much as possible, and use sunscreens regularly (see Prevention of Sunburn).

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