Diminution des concentrations plasmatiques de fentanyl par augmentation de son métabolisme hépatique par la rifampicine.Re: Genuinely Freaked Out!! Fentanyl OD or W/D, Unsure! Want Assistance!! I'm just a little involved for you. To me it seems like you might not be that opiod tolerant therefore you are experiecing some side effects that most peo