The warnings, on thousands of products sold in California, are stark.

“Use of the following products,” one label says, “will expose you to chemicals known to the State of California to cause cancer, birth defects or other reproductive harm.”

Now, new research shows the warnings may be working.

A study published Wednesday in the journal Environmental Science & Technology found that California’s right-to-know law, which requires companies to warn people about harmful chemicals in their products, has swayed many companies to stop using those chemicals altogether.

As it turns out, companies don’t want to sell a product that carries a big cancer warning label, said Dr. Megan Schwarzman, a physician and environmental-health scientist at the University of California, Berkeley School of Public Health and an author of the study.

Combine that with the threat of lawsuits and reputational costs, as well as companies just wanting to do the right thing for health, and “it becomes a great motivator for change,” she said.

California maintains a list of about 900 chemicals known to cause cancer and other health effects. Under the 1986 right-to-know law, also known as Prop 65, products that could expose people to harmful amounts of those chemicals must carry warning labels.

Critics had long mocked the measure, saying the warnings were so ubiquitous — affixed to cookware, faux leather jackets, even baked goods — that they had become largely meaningless in the eyes of shoppers. But the latest study found that companies, more than consumers, may be most influenced by the warnings.

To assess the law’s effect, researchers carried out interviews at 32 global manufacturers and retailers that sell clothing, personal-care, cleaning, and a range of home products. Almost 80 percent of interviewees said Prop 65 had prompted them to reformulate their products.

Companies can avoid the warning labels if they reduce the level of any Prop 65 chemicals below a “safe harbor” threshold.

A similar share of companies said they looked to Prop 65 to determine which chemicals to avoid. And 63 percent said the law had prompted them to also reformulate products they sold outside California.

The American Chemistry Council, which represents chemical manufacturers, did not immediately respond to a request for comment on the study.

No other state has a law quite like Prop. 65, requiring warnings on such a wide range of products about cancer or reproductive harm. New York enacted a more limited law in 2020 that requires manufacturers to disclose certain chemicals in children’s products and that bans the use of certain chemicals by 2023. Other states have laws geared toward disclosure of ingredients on labels.

California, meanwhile, is pushing ahead. A 2018 change to Prop 65 has meant products are starting to carry even more specific labels. Some food and beverage cans, for example, may carry labels that warn that they “have linings containing bisphenol A (BPA), a chemical known to the State of California to cause harm to the female reproductive system.”

The latest research is part of a larger effort to analyze Prop 65’s effect on people’s exposure to toxic chemicals. In a study published last year, researchers at the Silent Spring Institute and UC Berkeley found that in the years after certain chemicals were listed under the law, levels of those chemicals in people’s bodies decreased both in California and nationwide.

That research came with a caveat, however. In some examples where levels of a listed chemical decreased, a close substitute to that chemical, potentially with similar harmful effects, increased. Prop 65 has no mechanism to check the safety of alternative chemicals.

It suggested that stronger policies were needed at both the federal and state levels to study and regulate the thousands of chemicals on the market, Dr. Schwarzman said. “This is so much bigger than the individual consumer and what we choose off-the-shelf,” she said.