Chemist Sid Nelson and colleagues of the University of Washington, in Seattle, tested the effects of acetaminophen and caffeine on bacteria and found that caffeine triples the amount of a toxic byproduct, N-acetyl-p-benzoquinone imine, or NAPQI, an enzyme produced while breaking down acetaminophen that can cause liver damage and liver failure.

"People should be informed about this potentially harmful interaction," Nelson said in a statement. "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."

The bacteria used in the study were exposed to "megadoses" of both acetaminophen and caffeine -- higher than most individuals would normally consume -- but the toxic threshold has not yet been determined, the Chemical Research in Toxicology reported.

Nelson adds that the risks are also higher for those who take large amounts of medications that combine both acetaminophen and caffeine used for migraine headaches, arthritis and other conditions.