Fibroblast growth factor 14 (FGF14) is a member of the intracellular FGF (iFGFs) family and a functionally relevant component of the neuronal voltage-gated Na(+) (Nav) channel complex. Through a monomeric interaction with the intracellular C-terminus of neuronal Nav channels, FGF14 modulates Na(+) currents in an Nav isoform-specific manner serving as a fine-tuning regulator of excitability. Previous studies based on the highly homologous FGF13 homodimer crystal structure have proposed a conserved protein:protein interaction (PPI) interface common to both Nav channel binding and iFGF homodimer formation. This interface could provide a novel target for drug design against neuronal Nav channels. Here, we provide the first in-cell reconstitution of the FGF14:FGF14 protein complex and measure the dimer interaction using the split-luciferase complementation assay (LCA). Based on the FGF14 dimer structure generated in silico, we designed short peptide fragments against the FGF14 dimer interface. One of these fragments, FLPK aligns with the pocket defined by the β12-strand and β8-β9 loop, reducing the FGF14:FGF14 dimer interaction by 25% as measured by LCA. We further compared the relative interaction strength of FGF14 wild type homodimers with FGF14 hetero- and homodimers carrying double N mutations at the Y153 and V155 residues, located at the β8-β9 loop. The Y153N/V155N double mutation counteracts the FLPK effect by increasing the strength of the dimer interaction. These data suggest that the β12 strand of FGF14 might serve as scaffold for drug design against neuronal FGF14 dimers and Nav channels.