If your "cure" for a hangover involves popping lots of acetaminophen and chasing it with copious amounts of coffee, you could be harming your liver, says a U.S. study.

The study is published in the Oct. 15 print issue of the American Chemical Society journal Chemical Research in Toxicology.

Researchers found that caffeine triples the amount of a toxic byproduct, N-acetyl-p-benzoquinone imine, that the liver enzyme produces while breaking down acetaminophen.
(CBC)

Researchers at the University of Washington tested how acetaminophen and caffeine affect E. coli bacteria genetically engineered in the lab to express a key enzyme in the liver that detoxifies many prescription and nonprescription medications.

They found that caffeine triples the amount of a toxic byproduct, N-acetyl-p-benzoquinone imine (NAPQI), that the enzyme produces while breaking down acetaminophen.

NAPQI causes liver damage and can lead to liver failure in toxic interactions between alcohol and acetaminophen.

"People should be informed about this potentially harmful interaction," Sidney Nelson, one of the study's authors, said in a release. "The bottom line is that you don't have to stop taking acetaminophen or stop taking caffeine products, but you do need to monitor your intake more carefully when taking them together, especially if you drink alcohol."

"Individuals would have to drink upwards of 20 cups of strong coffee a day to have any increased risk of liver toxicity from acetaminophen," Nelson told CBCNews.ca.

"A very small group of individuals may be more at risk if they drink alcohol in excess, take high therapeutic doses of acetaminophen, and also drink lots of coffee or take drugs that combine acetaminophen and caffeine in high doses."

Nelson stresses that the bacteria used in the study were exposed to very high doses of both acetaminophen and caffeine — levels much higher than most people would normally consume on a daily basis. A toxicity threshold hasn't been determined.

Nelson also cautions that certain groups may be more vulnerable to the potentially toxic interaction than others, including those individuals who take certain anti-epileptic medications, such as carbamazepine and phenobarbital, and those who take St. John's wort, a popular herbal supplement used to treat depression.

These products have been shown to boost levels of the enzyme that produces NAPQI, an effect that will likely be heightened when taking both acetaminophen and caffeine together, he says.