Intralipid, a soybean oil-based lipid emulsion, is widely used in photomedicine to enhance light distribution due to its strong scattering properties. Although the optical characteristics of Intralipid are well documented, interactions with the reactive molecular species (RMS) generated during photodynamic therapy (PDT) and the impact of such interactions on therapeutic outcomes remain poorly understood. We reveal that Intralipid actively influences PDT response We examined how Intralipid affects the optical and photodynamic behavior of benzoporphyrin derivative (BPD), a clinical photosensitizer, in solution and across four ovarian cancer cell lines. The photodynamic properties of BPD, with and without Intralipid, were analyzed using fluorescence spectrometry and RMS probes, and PDT-induced oxidation of Intralipid components was characterized using LC-MS. The effects of Intralipid on BPD-PDT were evaluated under various conditions. Intralipid reduced BPD photobleaching and RMS generation, suggesting RMS quenching. Extensive oxidation of Intralipid components was observed following PDT. Finally, Intralipid significantly modified BPD-PDT efficacy across all four cell lines, depending on photosensitizer-light interval, dose, and incubation time. Intralipid acts as a bioactive modulator of PDT response, highlighting the need for further investigations both