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Certain compounds cause tissues in the body to react negatively to exposure to light, particularly sunlight and other sources of UV radiation. This type of reaction is known as phototoxicity. Areas of the body, most notably the skin and sometimes the eyes, can become extremely sensitive to light and the cells will sustain damage when they come in contact with it. This effect can be similar to but should not be confused with a photoallergic reaction, where the body has an allergic response to light-activated substances.
A variety of drugs can lead to phototoxicity; this includes both drugs that are ingested and those applied topically to the skin. Several types of antibiotics, including tetracyclines and sulfonamides, can cause extreme sensitivity to sunlight. Some non-steroidal anti-inflammatory drugs, or NSAIDs, may induce a phototoxic response. Retinoids used for skin care may make it more sensitive. Other drugs that may be a cause include some diuretics, antifungals, and neuoleptics.
The use of certain essential oils can also induce phototoxicity. The chemicals in some oils used in aromatherapy may make one's skin light sensitive and should therefore be used with caution. Bergamot oil in particular is known for leading to skin damage. Citrus oils such as lemon and lime are also common culprits. Some additional types that should be used with care are angelica root, lovage, and cumin.
Those suffering from phototoxicity typically experience discomfort or pain in the areas exposed to sunlight; the reaction leads to a breakdown of cell membranes or sometimes DNA in the cells, which in turn causes inflammation. The most common result is a severe sunburn, with the skin becoming very red and painful, and in some cases, the skin may even blister. The onset of the sunburn may occur very quickly, sometimes within minutes of exposure to the phototoxic substance and sunlight. There is also some concern that people exposed to phototoxic compounds long term may have more of a chance of developing skin cancer as well.
To test if a substance is phototoxic, scientists often use a process called the 3T3 neutral red phototoxicity test. This is an in vitro process where a dye called neutral red is applied to the compound in question. The reaction of the substance can then be assessed to determine if it will cause phototoxicity. Use of this test was popularized as an alternative to testing on animals.