A young woman with papules on the neck and axillae

Abstract

1 to 14 days after exposure. Photoallergic reactions are clinically similar to allergic contact dermatitis but require light to chemically alter the medication or chemical to render it allergenic. They occur more frequently with topically applied agents, such as oxybenzone, than with systemic medications. The histopathologic changes of photoallergic reactions are identical to contact dermatitis with spongiosis in the epidermis and perivascular infiltrate of lymphocytes in the dermis. Many drugs induce photosensitivity. Examples include quinolones, tetracyclines, diuretics, nonsteroidal antiinflammatory drugs (NSAIDs), and others. The ability of a drug to act as a photosensitizer depends on its physical, chemical, pharmacokinetic, and pharmacodynamic properties. These include a low molecular weight (200-500 Daltons), planar, tricyclic, or polycyclic configurations, heteroatoms in their structure to enable resonance stabilization, and aromatic halogen substituents. The absorption maximum refers to the ability of the chemical to absorb light. The lipophilicity of a drug is an important factor in its ability to reach the skin. Doxycycline has a molecular weight of 444.4 Daltons, a tetracyclic configuration, and does not contain heteroatoms or aromatic halogen atoms. Its absorption maximum is 352 nm, which is in the UVA spectrum. The photosensitivity effect created by doxycycline is dependent on the medication dose, UVA intensity, and the skin phenotype. Demeclocycline is the strongest photosensitizer of the tetracycline group; however, it is rarely used. Tetracycline is also known to cause phototoxic reactions, but less so than doxycycline. Minocycline is minimally phototoxic. Like sunburn, phototoxic reactions usually subside within a week. Treatment is symptomatic. Avoiding the offending agent and excessive sunlight exposure is required. It is important to apply sunscreen 15 minutes before exposure and repeat at least every 2 hours. If excessive sun is difficult to avoid—as in a wintertime cruise to a high UV-light destination—discontinue the medication for several days and restart. Of note, window glass transmits sunlight above 330 nm, which is in the UVA range, and artificial room lighting contains wavelengths above 370 nm. For this series, the recommended choices are: 1, d; 2, c; 3, a.