67 failures at extracting acetaminophen from Excedrin(R)
Did you notice any change of colour (e.g. to brown) and/or the formation of gunk upon adding the 3M NaOH?

I'd assume one would have to boil it, but is the following scenario conceivable under your conditions:


alkaline hydrolysis of the acetaminophen (paracetamol) to p-aminophenol
aerial oxidation to the corresponding quinone imine
partial polymerisation of the latter


You have confirmed that acetaminophen was initially there. A a change in the constitution of Excedrine since the publication of the article can thus be excluded. Isn't a reaction (hydrolysis or whatever) under your extraction not the most likely explanation for the vanished acetaminophen?