Fluorouracil or 5-FU is a pyrimidine analog which is used in the treatment of cancer. It is a suicide inhibitor and works through irreversible inhibition of thymidylate synthase. When applied topically, it selectively kills UV-damaged skin cells. It is used primarily to treat actinic keratoses. A patient handout is available.
4% fluorouricil in peanut oil once daily for 2 weeks in one study was as efficacious with slightly fewer side effects compared with 5% BID [J Drugs Dermatol 2016;15(10):1218].
A combination of 5-FU 0.5%/salicylic acid 10% used once daily for 12 weeks is being studied [Dermatol Ther (Heidelb). 2017;7:81-96].
Obviously, skin crusting, inflammation and pain are expected side effects when 5-FU is applied topically. Occasionally patients are allergic to 5-FU. The redness and inflammation with ACD is confluent and not patchy or reticulate as with the usual skin inflammation.
A man in his 60's developed unsteadiness on his feet, altered coordination in gait and dizziness 3.5 weeks into the use of 5-FU BID on the scalp. Cessation of the medication resolved the symptoms within 8 hours [AD 2017;153;831].
Topical 5-FU can induce melanosis of the nail. This child was treated with topical 5-FU for periungual warts and developed nail pigmentation near the proximal nail fold. The pigment has grown out now over several months and will soon be shed.
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