Basic 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.
These 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.
Photoallergic 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.
None 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.
This is the same as for sunburn. Drugs should be stopped if further exposure to ultraviolet light is likely.