One of the most popular drugs in the U.S. could soon be declared a carcinogen in California.

Acetaminophen is an active ingredient found in more than 600 prescription and over-the-counter pain relievers, fever reducers, sleep aids and cough, cold and allergy medicines. It's the most commonly used pain medication in the country, according to WebMD. It's found in medications from well-known brands including Tylenol, Excedrin, Theraflu and Robitussin.

The drug — which is known outside the U.S. as paracetamol — is on California's list of drugs under review because of its potential links to cancer.

A California law known as Proposition 65 requires that the state warn residents about chemicals that may cause cancer or reproductive toxicity. This spring, governor-appointed scientists will conduct a public hearing to decide whether acetaminophen will be added to the list. Currently, there are about 900 chemicals — including alcoholic beverages, flame retardants and pesticides — on the list.

California regulators hare reviewed 133 studies on acetaminophen that were all published in peer-reviewed journals, according to the Associated Press. Findings were mixed, with some reporting an increased risk of various types of cancers, while others found none. The review found that it's hard to isolate the ingredient from other cancer-causing variables like smoking, so acetaminophen is difficult to study.

'A difficult issue'

Acetaminophen was evaluated by the International Agency for Research on Cancer (IARC) in 1990 and 1999 and after each review, the agency said there was not enough evidence to list it as a carcinogen.

Now, the the U.S. Food and Drug Administration (FDA) has told state officials that labeling acetaminophen as cancer-causing would be "false and misleading" and illegal under federal law.

Critics argue that California regulators are confusing consumers with the growing list of warning labels. Supporters, however, say the oversight is necessary for safety.

"It's a difficult issue because it's a very commonly used drug. But that doesn't make any difference. That's not what our mandate is," Thomas Mack, chairman of the Carcinogen Identification Committee (CIC) and a professor of preventive medicine at the University of Southern California, told the AP.

Acetaminophen's popularity

medications on store shelves Acetaminophen is found in more than 600 different medications. (Photo: Sheila Fitzgerald/Shutterstock.com)

There's a good chance a drug in your medicine cabinet contains acetaminophen in some form. According to the Consumer Healthcare Products Association, each week, about 23% of adults in the U.S. use a medicine that contains acetaminophen.

When taken as directed, the drug is "generally safe and effective," says WebMD. However, the FDA points out that acetaminophen can cause serious liver damage if you use more than the directions call for.

Unlike other common pain relievers, such as nonsteroidal anti-inflammatory drugs (NSAIDS), acetaminophen doesn't increase the risk of heart or stomach problems, points out Medical News Today. That's why it's often the first choice of pain reliever for people who can't take other medications.

Last fall, the California Rheumatology Association (CRA) wrote a letter to the California committee, urging the group not to list acetaminophen as a carcinogen. The letter points out that the medicine is important for those who can't take NSAIDs but don't need stronger opioids.

"The decision before the CIC is an important public health issue that is of enormous consequence: getting it wrong could create significant consumer confusion, unnecessary fear, and potential harm," CRA President Samy Metyas, M.D. writes. "Given the current pain management landscape in the U.S., it is counter productive to be unnecessarily frightening people away from using safe, effective, and trusted pain medications like acetaminophen."

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

Should you avoid acetaminophen?
California considers labeling acetaminophen a carcinogen.