Taking slightly too much paracetamol day after day can be fatal, experts have warned.
A dangerous dose might just be a few pills too many taken regularly over days, weeks or months, they said.
Researchers at Edinburgh University saw 161 cases of "staggered overdose" at its hospital over a 16-year period.
People taking tablets for chronic pain might not realise they were taking too many or recognise symptoms of overdose and liver injury, they said.
The researchers told the British Journal of Clinical Pharmacology that this life-threatening condition could be easily missed by doctors and patients.
Doctors may not initially spot the problem because blood tests will not show the staggeringly high levels of paracetamol seen with a conventional overdose, where someone may have swallowed several packets of the drug.