photochemical laws require a drug to absorb UVR to cause such a reaction. Most
drugs listed in Table 16.2 absorb UVA as well as UVB, and so window glass, protective
against sunburn, does not protect against most phototoxic drug reactions.
Tenderness and redness occur only in areas exposed both to sufficient drug and to sufficient UVR (Fig. 16.4). The signs and symptoms are those of sunburn. The skin may later develop a deep tan.
reactions are not immunological. Everyone exposed to enough of the drug, and to
enough UVR, will develop the reaction. Some drugs that can cause phototoxic
reactions are listed in Table 16.2. In addition, contact with psoralens in
plants (Fig. 16.5) can cause a localized phototoxic dermatitis
(phyto-photodermatitis; Fig. 16.6). These areas burn and may blister, leaving
pigmentation in linear streaks and bizarre patterns.
reactions are difficult to distinguish; the more so as the same drugs can often
cause both pho-toallergic and phototoxic reactions. The main differ-ences
between phototoxicity and photoallergy are shown in Table 16.3.
are usually required. In difficult cases, photo-testing can be carried out in
special centres. The action spectrum (the wavelengths that cause the reaction)
may incriminate a particular drug.
is the same as for sunburn. Drugs should be stopped if further exposure to
ultraviolet light is likely.