Acetaminophen carries with it a number of risks, according to the U.S. Food and Drug Administration, making the rare move of warning prescribers about something it had warned them about only four months ago. The FDA reminded physicians and other prescribers of pharmaceuticals to stop writing prescriptions for, stop dispensing prescriptions for, and stop taking prescription medications with more than 325 milligrams of acetaminophen.

The warning was given because of of the risks of liver failure. Acetaminophen overdose is now worse than viral hepatitis infection as a cause of acute liver failure. It is now the second most common cause of liver failure requiring transplantation in the United States. It’s a reminder that acetaminophen can have a number of side effects. According to Harvard Medical School’s Family Health Guide, most acetaminophen is broken down into harmless substances removed from the body in urine. However, a small percentage is rendered into a compound that can be extremely harmful to cells.

One of the potential dangers after taking even conservative dosages of acetaminophen are Stevens Johnson Syndrome (SJS) and toxic epidermal necrolysis (TEN). SJS and TEN are deadly allergic reactions, typically beginning with a rash and flu-like symptoms, followed by severe blistering and skin detachment. Although the condition is rare, the FDA warns patients taking acetaminophen to seek medical attention if they develop a rash or other skin reaction.

The FDA doesn’t know how frequently SJS occurs, but it is quite rare, and is requiring that companies who sell over-the-counter acetaminophen products, other than oral medication, add an SJS warning to the labels.

It is now the general scientific consensus that SJS and TEN are triggered by over-the-counter medications like acetaminophen, although it is extremely hard to predict who is susceptible to the conditions, which are both characterized by purple blistering around the body. SJS can cover between 10 percent and 30 percent of the body’s surface area, while TEN can cover up to 90 percent.

TEN is a latent form of SJS, and typically occurs when the condition spreads at a more rapid pace than normal. Stevens Johnson Syndrome usually presents within the first two to four weeks after starting the medication.