A warning tonight about popular pain relievers. How too much acetaminophen can take a serious toll. And now tylenol is taking action. Here's abc's linsey davis. Tylenol believes the right pain reliever for you is one that works with your body. Reporter: It's advertised as tough on pain, easy on your stomach. But it's what tylenol might do to your liver that has the manufacturer issuing the new warnings. Johnson & johnson says starting in october, the caps of new bottles of extra strength tylenol will have this message in big red print -- contains acetaminophen and always read the label. Extra strength tylenol tablet is 500 milligrams. So, you could take eight of those every day, spaced out properly, and you'd still be within the safe range. Reporter: The pain relieving ingredient is the nation's leading cause of sudden liver failure. And as 33-year-old riley McDERMOTT LEARNED FIRST HAND, IT Isn't just found in tylenol. If my fiance gotten me to the e.R. Half an hour later, I'd be dead. Reporter: After inadvertently overdosing on a combination of prims codeine for an ankle injury and another medicine to help her sleep, she ultimately ended up on the liver transplant list. You're in pain and you take something and you just don't even think about it building up in your body. Reporter: Liver damage is extremely rare. The more common problem is people overdosing on the drug, which sends as many as 80,000 people in the u.S. To the e.R. Each year. In a statement to abc news, johnson & johnson said, "the new cap message is designed to help encourage appropriate acetaminophen use." Riley would agree. In addition to reading the label, it's critical to follow the dosage. Linsey davis, abc news, new york.