The back-to-back reports represent a major dilemma for the millions of people who require frequent pain relief and who have switched from acetylsalicylic acid (ASA) to acetaminophen because it was thought to be safer.

No one should stop using acetaminophen as a result of the two studies, [Paul K. Whelton] concluded - although people should certainly be switched away from it at the first sign of kidney problems.

They found that the risk of kidney failure was about 1.4 times normal for individuals who took from two to seven acetaminophen tablets per week, rising to 2.1 times normal for those who took more than one tablet a day.