Sun-Sensitive Drugs
(Photosensitivity to Drugs)
Medical Author: Siamak Nabili, MD, MPH
Medical Editor: Melissa Conrad Stöppler, MD
Making Sense of Sunscreen Products
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor:
Barbara
K. Hecht, PhD
Most people are understandably confused when it comes to
choosing a sunscreen
because of the baffling array of available choices. Common questions about
sunscreens include
- How high should the SPF be?
- Should it block UVA or UVB?
- Does it matter whether it is a gel, cream, or spray?
- Should it be water-resistant or waterproof?
SPF stands for sun protection
factor.
The SPF numbers on a product can range from as low as 2 to as high as 60. These
numbers refer to the product's ability to screen or block out the sun's burning
rays. The SPF rating is calculated by comparing the amount of time needed to
produce sunburn on protected skin to
the amount of time needed to cause a sunburn on unprotected skin. The higher the
SPF, the greater the sun protection.
What is photosensitivity?
Photosensitivity (or sun sensitivity) is inflammation of the skin induced by
the combination of sunlight and certain medications or substances. This causes
redness (erythema) of the skin and may look similar to sunburn.
Generally, these reactions can be divided into two mechanisms, 1) phototoxic
reactions and 2) photoallergic reactions. Phototoxic drugs are much more common
than photoallergic drugs.
What is the difference between a photoallergic and a phototoxic reaction?
Phototoxic reactions
In phototoxic reactions, the drug may become activated by exposure to
sunlight and cause damage to the skin. The skin's appearance resembles sunburn,
and the process is generally acute (has a fast onset). Ultraviolet A (UVA)
radiation is most commonly associated with phototoxicity, but ultraviolet B
(UVB) and visible light may also contribute to this reaction.
A phototoxic reaction typically clears up once the drug is discontinued and
has been cleared from the body, even after re-exposure to light.
Photoallergic reactions
In photoallergic reactions, the ultraviolet exposure changes the structure of
the drug so that is seen by the body's immune system as an invader (antigen).
The immune system initiates an allergic response and cause inflammation of the
skin in the sun-exposed areas. These usually resemble eczema and are generally
chronic (long-lasting). Many drugs in this family are topical drugs.
This type of photosensitivity may recur after sun exposure even after the
drug has cleared from the system.
What is ultraviolet light?
Ultraviolet (UV) light is radiation energy in the form of invisible light
waves. UV light is emitted by the sun and by tanning lamps.
The sun discharges three types of ultraviolet radiation:
- ultraviolet A (UV-A),
- ultraviolet B (UV-B), and
- ultraviolet C (UV-C).
Only UV-A and UV-B rays
reach earth. (UV-C does not penetrate the earth's upper atmosphere.)
Tanning lamps also produce UV-A and/or UV-B. These artificial rays affect the
skin in the same way as do UV-A and UV-B from the sun.
Next: What are the symptoms of sun sensitivity
(photosensitivity)? »
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