The symptoms in the OP are genuinely ambiguous, and I think about this a lot bc injection day creates a confounding window that’s hard to parse. Two distinct things get conflated here: Endotoxin / excipient reaction: Compounded tirz introduces variables brand doesn’t - BAC water quality, vial sterility, lipid excipients. Endotoxin contamination produces exactly this profile: chills, myalgia, nausea 4-8h post-injection, usually resolving in 12-24h. If it recurs from the same vial, that’s informative. Coincidental viral onset: Standard flu incubation is 1-4 days. Exposed Sunday, injected Monday, symptoms Monday night looks like a reaction but isn’t. The overlap window is wide enough to fool you. What I actually do: log injection time and symptom onset time, and note if anyone in your orbit goes down in the same 72h window. That’s real signal. The only clean A/B test is two injections from a different vial. If symptoms don’t recur, you have your answer. If they do, stop using that source.