NEARLY one in four patients admitted to a Scottish hospital with paracetamol-induced liver failure accidentally overdosed on the drug while trying to treat common ailments such as headaches, toothache and stomach pain.

Researchers found they fell victim to "staggered" intakes of of the painkiller and were unaware of the danger they were putting themselves in.

Experts analysed data from 663 patients admitted to Edinburgh Royal Infirmary between 1992 and 2008 with liver damage caused by excessive paracetamol consumption. They found 24% overdosed by taking slightly too much of the drug on two or more occasions over an eight-hour period before their admission to hospital.

The study, published today in the British Journal of Clinical Pharmacology, warned these patients are at greater risk of dying than those who overdose in a single, massive intake. But they warn that existing tests are not effective in identifying them on admission to hospital.

The clinical threshold for a paracetamol overdose is classed as 4g or more in a single intake – equivalent to around eight tablets.

Around two-thirds of those who had taken a staggered overdose had done so with no intention of committing suicide, and due to the more gradual build-up of toxins in the blood tended to seek medical help later – usually more than 24 hours after their most recent paracetamol intake. This makes treatment more difficult.

The study said: "Patients with staggered paracetamol overdoses had reduced survival compared with single intentional overdoses, despite ingestion of significantly lower total amounts of paracetamol."

Study author Dr Kenneth Simpson, a consultant at the ERI's Scottish Liver Transplant Unit and Edinburgh University academic, said the potentially-fatal overdoses were all the more dangerous because the patient was unaware of what was happening. He 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."

Part of the problem is doctors normally assess how much danger a patient is in when they arrive at hospital by taking a blood sample and finding out how much paracetamol is present. In the case of a single-dose overdose, the sample gives valuable information, but people with staggered overdoses may have low levels of paracetamol even though they are at a high risk of liver failure and death.

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

The situation was worse for those who arrived at hospital more than a day after taking an overdose, he added.

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

Dr Simpson said while cases of patients requiring transplants from paracetamol-induced liver damage were rare, cases of staggered overdose were becoming increasingly common. He also warned anyone combining it with other drugs should check these did not contain paracetamol.