Background: We analyze the safety and tolerability of trofinetide and provide a preliminary evaluation of its efficacy in adolescent and adult males with fragile X syndrome. Methods: This study was an exploratory, phase 2, multicenter, double-blind, placebo-controlled, parallel group study of the safety and tolerability of orally administered trofinetide in 72 adolescent and adult Conflicts of interest: E.B.-K. participated in the Neu-2566-FXS 001 trial, has been a consultant for Seaside Therapeutics, Novartis, Roche, Alcobra, Neuren, Cydan, Fulcrum, GW, Neurotrope, Marinus, Zynerba, BioMarin, and Ovid Pharmaceuticals regarding conduct of clinical trials in FXS and other rare neurological diseases, is on the Scientific Advisory Board for Vtesse/Mallinckrodt to provide guidance on clinical trials in Niemann-Pick type C (NP-C), has received research funding to conduct clinical trials in FXS from Novartis, Roche, Seaside Therapeutics, Alcobra, Neuren, Ovid, and Zynerba to conduct a clinical trial in NP-C from Vtesse/ Mallinckrodt, and to develop FMR1 testing standards from Asuragen. She was also a participating principal investigator on a clinical trial in Rett syndrome sponsored by Neuren Pharmaceuticals. She has research finding through grants from NIH, CDC, FRAXA Research Foundation, International Rett Syndrome Foundation, and the Phelan McDermid Foundation. She was consultant to GCC on the project funded by Neuren Pharmaceuticals for the psychometric analysis of the FXRS. J.P.H. was an employee at Neuren Pharmaceuticals, Inc. during the conduct of the research. He is currently an employee at AMO Pharma Ltd., which was not involved in the clinical trial reported in this manuscript. N.T. participated in the Neu-2566-FXS 001 trial and has received research funding from NICHD and CDC, and for industrysponsored clinical trials in autism and/or fragile X from Seaside Therapeutics, Roche, Neuren, Alcobra, Zynerba, and Ovid. She has consulted with Zynerba and Ovid on clinical trial design and outcome measures in FXS. R.H. participated in the Neu-2566-FXS 001 trial and receives research funding from NICHD, Azrieli Foundation, Curemark, and Zynerba currently regarding studies in FXS, autism, or premutation involvement. She has consulted with Zynerba,