Research published in the British Journal of Clinical Pharmacology showed that although ingesting less of the drug overall, people taking “staggered overdoses” were about a third more likely to die.
They also had a greater chance of liver and brain problems, and were more likely to need kidney dialysis or assistance with breathing, especially if they had waited at least a day before going to hospital.
Dr Kenneth Simpson of Edinburgh University, who led the study, said: “They haven’t taken the sort of single-moment, one-off massive overdoses taken by people who try to commit suicide, but over time the damage builds up, and the effect can be fatal.
“The problem is that some people were taking regular paracetamol and not appreciating that they should stick to 4g in a day.
“They were sometimes taking two preparations, both of which contained paracetamol, such as regular paracetamol as well as headache tablets.”
Researchers studied 663 patients admitted to the Royal Infirmary of Edinburgh for severe, paracetamol-induced liver injury and found that a quarter had taken staggered overdoses – meaning two or more doses, more than eight hours apart adding up to an amount above the daily limit.
The average staggered overdose was 48 tablets – slightly lower than the average one-off overdose of 54 tablets – but the staggered doses could have been taken over a period of up to a week.
In some cases patients had taken two large doses within a 24-hour period but in others they had just two or three extra pills a day over the course of four or five days, Dr Simpson explained.
While one third of people taking staggered overdoses had been attempting suicide, about half had simply been self-medicating for conditions like joint and muscular pains or toothache, he added.
The study also showed that patients taking staggered overdoses were older, with an average age of 39, and more likely to have been abusing alcohol.
Dr Neil Kitteringham, from the University of Liverpool’s MRC Centre for Drug Safety Science, said: “Paracetamol overdose is a significant burden to the NHS.
“This large study from Edinburgh shows that unintentional overdosing with paracetamol may have more serious consequences than a single overdose taken with suicidal intent.”