 Low-level laser therapy, LLLT, has been controversial since its discovery in 1967 due to a lack of understanding of basic mechanisms and improper dosimetric parameters leading to negative studies. In vitro and animal experiments have shown that LLLT's biphasic dose response or Arne-Schultz curve is mediated by various factors, including adenosine triphosphate, ADP, and mitochondrial membrane potential. The Janus nature of reactive oxygen species, ROS, which can act as a beneficial signaling molecule at low concentrations but a harmful cytotoxic agent at high concentrations, may partly explain the observed responses in vivo. Transcranial LLLT for traumatic brain injury, TBI, in mice shows a distinct biphasic pattern with peaks in beneficial neurological effects when the number of treatments and energy density are varied. Further research is needed to optimize clinical treatments based on biphasic dose responses. This article was authored by Ying Ying Huang, Soba Kei Sharma, James Carroll, and others. We are article.tv, links in the description below.