Reaching for the paracetamol alongside your morning coffee may be bad for your health, researchers say.

A study indicated that a combination of large quantities of the pain-killer and caffeine appeared to increase the risk of liver damage. Scientists found that caffeine tripled the amount of a toxic by-product created when paracetamol was broken down.

However, the University of Washington team so far has plied only bacteria and rats with large doses. British scientists emphasised that far more research would be needed to prove any danger to humans.

US researchers, writing in the journal Chemical Research in Toxicology, recommend that people should limit the amount of coffee or energy drinks they consume while taking paracetamol. Even relatively small overdoses of paracetamol can cause permanent damage to the liver. Scientists already know that heavy alcohol consumption can make the drug even more toxic, but this is the first suggestion that combining paracetamol and caffeine could produce a similar effect.

Caffeine is added to many commercially available paracetamol tablets as it is believed that this increases their effects.

Sidney Nelson, who led the study, said: "You don't have to stop taking acetaminophen [paracetamol] or stop taking caffeine products, but you do need to monitor your intake more carefully when taking them together, especially if you drink alcohol." The study used E. coli bacteria that had been modified genetically to produce a key liver chemical which, in humans, helps the body to break down paracetamol.

When the bacteria were exposed to very large doses of paracetamol and caffeine together, the amount of the toxic by-product produced was tripled. This is the toxin that causes liver damage after a paracetamol overdose.

Dr Nelson said that the quantities of caffeine and paracetamol used in the study were far higher than most people would consume daily but added that the amount needed to produce a harmful effect in humans had not been calculated. Previous studies showing that high doses of caffeine can increase the severity of liver damage in rats with paracetamol-induced liver damage support this finding.

Some people are thought to be more vulnerable than others. These include those taking antiepileptic medication or St John's wort, which have been shown to boost levels of the enzyme involved. People who drink a lot of alcohol are also at higher risk because it can trigger another enzyme that produces the liver toxin.