One of the most commonly used drugs on the market may be declared a carcinogen by California.

Acetaminophen, an active ingredient in popular pain relief medications such as Tylenol, Excedrin and Midol, has been on the state’s list of drugs under review for years because of tenuous links to cancer.

In the spring, a panel of scientists appointed by the governor will conduct a public hearing to determine whether acetaminophen – known in other countries as paracetamol – will be added to a list of about 900 chemicals the state considers a cancer risk. A California law called Proposition 65 requires the state to warn its residents about chemicals that may cause cancer or reproductive toxicity.

More than 100 studies published in peer-reviewed journals have yielded mixed results on the question of whether acetaminophen increases the risk of some forms of cancer. An analysis of those studies by state regulators pointed out it’s difficult to isolate the drug from other cancer-causing factors, such as smoking.

“With the data the way it is, I would be surprised if they added it,'' said David Reeves, an associate professor at Butler University in Indianapolis and a clinical pharmacy specialist at a local hospital. "It depends on what the makeup of the committee is. If you have healthcare professionals on that committee, people who understand these studies, I would think they’d probably be hesitant to put that type of scarlet letter on acetaminophen.’’

The International Agency for Research on Cancer reviewed acetaminophen in 1990 and 1999, and at neither time did it list the drug as a carcinogen. In addition, the U.S. Food and Drug Administration has told state officials that labeling acetaminophen as cancer-causing would be “false and misleading” and illegal under federal law.

A statement from Tylenol maker Johnson & Johnson said the company stands by its product and does not believe it should be included on the list, which wouldn't ban the drug but would require the carcinogen warning.

"In order to be listed under Prop 65 in California, acetaminophen must be ‘clearly shown through scientifically valid testing’ to cause cancer,'' the statement said. "Extensive data generated through epidemiologic, genotoxicity, and animal carcinogenicity studies do not support a conclusion that there is a causal relationship between acetaminophen and cancer.''

But Sam Delson, spokesman for the California Office of Environmental Health Hazard Assessment, said the panel of scientists decided in 2011 that there was enough evidence of elevated cancer risk that considering the addition of acetaminophen to the list should be given “high priority.’’

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Critics have accused state regulators of overreaching and confusing consumers, noting that California has the biggest list of carcinogenic chemicals in the nation. State officials counter by saying they’re duty-bound to uphold the law.

“It's a difficult issue because it's a very commonly used drug,’’ said Thomas Mack, chairman of the Carcinogen Identification Committee. “But that doesn't make any difference. That's not what our mandate is.”

According to WebMD.com, acetaminophen is the most popular pain relief medicine in the U.S., and it’s considered safe and effective with proper use. The drug, also used to reduce fever, can be found in more than 600 prescription and over-the-counter medications.

But the FDA warns on its website that misuse of acetaminophen is dangerous and says that “taking more than the recommended amount can cause liver damage, ranging from abnormalities in liver function blood tests, to acute liver failure, and even death.’’

Reeves said the majority of studies have not found an association between acetaminophen and cancer. Those that have pointed to a possible link with renal cancer and blood deficiencies.

For Reeves, the biggest concern is what options may be left for patients who get scared away from acetaminophen. He said other painkillers like ibuprofen or naproxen are not viable alternatives for some patients, like those who've had ulcers or are on blood thinners.

“If you can’t take those and you can’t take Tylenol and you have something like arthritis, where you have frequent pain, your next option is going toward the opioids,'' Reeves said. "And with the opioid epidemic the way it is … I don’t know if the consequences are worth it.’’

Contributing: The Associated Press