A user reported a severe acute toxic reaction after taking a 500mcg dose of ACE-031, despite having no issues with smaller 100mcg doses. Within 15 minutes of the higher dose, the user experienced intense flu-like symptoms, including muscle aches, nausea, chills, vomiting, and diarrhea that required urgent care. The user noted the product had a Certificate of Analysis showing 99% purity, leading to concerns about potential endotoxin contamination.