Making a habit out of taking paracetamol (acetaminophen) with your morning coffee could have an unexpected side effect: permanent liver damage.

Researchers at the University of Washington published a study in the journal Chemical Research in Toxicology, warning that consuming large quantities of caffeine and paracetamol (acetaminophen) can raise the risk of liver damage.

The authors found that caffeine tripled the amount of a toxic by-product created when the organism breaks the paracetamol down. As their experiment was conducted on genetically modified bacteria and rats, the authors say more research is necessary to ascertain the danger for human health.

The study used E. coli bacteria that had been modified genetically to produce an essential liver enzyme which, in humans, helps the body to break down paracetamol.

When exposed to very large doses of paracetamol and caffeine together, the bacteria produced three times as much toxin – a toxic by-product produced during the breaking down of paracetamol.

This toxin causes liver damage after a paracetamol overdose.

Dr. Sidney Nelson, who led the study, noted that humans do not usually ingest such large quantities of caffeine and/or paracetamol and emphasized that more research is necessary in order to discover what the harmful doses are for humans.

He also said that people should nevertheless be careful about the amounts of caffeine and paracetamol they consume, as well as avoid taking the two together.

Dr. Nelson 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.”

Previous studies had already shown that alcohol consumption poses a higher risk of liver damage for people who take paracetamol. The University of Washington team says this is the first study that looks at the potentially harmful side effects of mixing caffeine with paracetamol.

Caffeine is often added to commercially available paracetamol, because it enhances the effects of the painkiller.