Little overdose of paracetamol taken repeatedly over some time can put a person at the risk of dying. The risk is greater in case of a multiple overdose than in a single overdose, a new study says.

Such people do not come to the hospital reporting overdose but because they feel unwell. According to the study, these people need to be identified and treated at the earliest.

The study has been carried out by scientists from the University of Edinburgh and the Scottish Liver Transplantation Unit, Scotland. The results appeared in the British Journal of Clinical Pharmacology on Tuesday.

This kind of overdose or staggered overdose can occur when people have pain and repeatedly take a little more paracetamol than they should, scientists 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," Dr Kenneth Simpson, the scientist who led the study, said.

Doctors normally assess danger to a patient by testing how much paracetamol is present in the blood. In the cases of single overdose, the blood sample gives valuable information, but people with staggered overdoses may have low levels of paracetamol in their blood even though they are at a high risk of liver failure and death, scientists said.

The team analysed data from 663 patients who had been admitted to a hospital between 1992 and 2008 with paracetamolinduced liver injury. They found that 161 had taken a staggered overdose, usually to relieve common pains such as abdominal or muscular pains, headache and toothache.

"On admission, staggered overdose patients were more likely to have liver and brain problems, require kidney dialysis or help with breathing and were at a greater risk of dying than people who had taken single overdoses," Simpson said.

The outcome for people coming to a hospital more than a day later taking a single overdose was also bad and they were also at a high risk of dying or needing a liver transplant, the scientists said.

"Staggered overdoses or patients presenting themselves late after an overdose need to be closely monitored and considered for the paracetamol antidote, N-acetylcysteine, irrespective of the concentration of paracetamol in their blood," Simpson said.

Since measuring paracetamol in the blood is a poor assessment for a patient's status in cases of staggered overdoses or delayed presentation, doctors must find new ways of assessing whether a patient can be sent home, need medical treatment to counteract the paracetamol, or need to be considered for a liver transplant.