Controllable self-assembly and understanding of the interaction between single metabolite fibrils and live-cell membranes have paramount importance in providing minimal treatment in several neurodegenerative disorders. Here, utilizing the nonlinear nature and peculiar hydrogen bonding behavior of the dimethyl sulfoxide (DMSO)-water mixture, the selective self-assembly of a single metabolite 5-fluorouracil (5-FU) is achieved. A direct correlation between water availability and selective self-assembly of 5-FU is ratified from the excited-state dynamics. The specific fibrillar structures of 5-FU exhibit a great potential to modulate live cell membrane fluidity and model membrane lipid distribution. After 5-FU fibril addition, a disorder of H-bonded water molecules arises several layers beyond the first hydration shell of the polar headgroups, which essentially modifies interfacial water structure and dynamics. Overall, our results shed light on the role of solvent to govern specific self-assembly and also lay the foundation accounting for the earlier stage of several diseases and multidrug resistance.