In a controlled clinical trial, 12 white men with normal skin applied 1% fluorouracil in propylene glycol to the medial aspect of the left upper arm twice daily for 14 days. At the end of the treatment period, a 4-mm punch biopsy was taken from the center of the treated area. A similar 4-mm biopsy was taken from the opposite untreated arm. Biopsy specimens were randomly coded "A" and "B" in a double-blind fashion, and stored in buffered Formalin until evaluated. This study confirms previous conclusions that 1% fluorouracil does not affect nonkeratotic skin.
Simmonds WL. Histologic Effect of Fluorouracil on Normal Skin. Arch Dermatol. 1969;100(5):608-609. doi:10.1001/archderm.1969.01610290092021