Well, acetaminophen (paracetamol) in higher doses (2g+) is metabolized by P450-2D6 , which is also the cytochrome ideally involved with MDMA metabolism. I say ideally because some people have a 2D6 deficiency, and so metabolize these drugs using different 'mixed function oxidases' and/or enzymes.
Toxicity from large amounts of panadol occurs because enzymes involved in other preferred routes (least toxic) become saturated. 2D6 then steps in and produces a toxic intermediate (N-acetyl-B-benzoquinone imine ) The intermediate then goes one of 2 ways, one of which is dependant on glutathione. If glutathione becomes depleted, the alternative pathway leads to cellular death.
Now, you might think, mmm...if MDMA occupies 2D6, then this nasty metabolic pathway will be unavailable and so no toxic intermediate is formed. Well that could be perhaps what happens, but more likely is that the body will employ another mixed function oxidase ...) they usually love to fill in for one another :D ) This may or may not increase liklihood of toxicity, but a quick search among the journals didn't find anything.
I know of people who use panadol when rolling, and none seem to have so far come to any harm. However any liver problems and liver damage may not show up for years, so tread carefully if a headache is persistent. Look for causes if this occurs regularly ie dehydration, blood pressure increase, alcohol consumption etc.
