ICH E6(R3) formally relocated Investigator's Brochure requirements into Appendix A, effective July 2025 in the EU and finalized by FDA in September 2025. This structural change signals that the IB is now positioned as an integral component of the Good Clinical Practice framework alongside the protocol. For sites, the IB matters beyond box-checking: it is the document investigators reach for when an unexpected event occurs mid-trial. Its Reference Safety Information section determines whether a serious adverse reaction is unexpected and qualifies as a SUSAR. Under 21 CFR 312.55, sponsors must provide an IB before trial start and keep investigators informed of new observations, particularly adverse effects. The EU CTR requires annual review and updates when new safety information emerges. Sponsors who drafted IBs under E6(R2) and have not revisited their templates should do so now. The AI medical writing market supporting regulatory documents like IBs was valued at $869 million in 2024 and is projected to reach $1.76 billion by 2030. IB maintenance is enforceable through FDA's BIMO program, and violations in distribution or updates can result in Form 483 citations during inspections.