The Food and Drug Administration on Thursday approved a birth control pill to be sold without a prescription for the first time in the United States, a milestone that could significantly expand access to contraception.

The medication, called Opill, will become the most effective birth control method available over the counter — more effective at preventing pregnancy than condoms, spermicides and other nonprescription methods. Experts in reproductive health said its availability could be especially useful for young women, teenagers and those who have difficulty dealing with the time, costs or logistical hurdles involved in visiting a doctor to obtain a prescription.

The pill’s manufacturer, Perrigo Company, based in Dublin, said Opill would most likely become available from stores and online retailers in the United States in early 2024.

The company did not say how much the medication would cost — a key question that will help determine how many people will use the pill — but Frédérique Welgryn, Perrigo’s global vice president for women’s health, said in a statement that the company was committed to making the pill “accessible and affordable to women and people of all ages.” Ms. Welgryn has also said the company would have a consumer assistance program to provide the pill at no cost to some women.

“Today’s approval marks the first time a nonprescription daily oral contraceptive will be an available option for millions of people in the United States,” Dr. Patrizia Cavazzoni, director of the F.D.A.’s Center for Drug Evaluation and Research, said in a statement. “When used as directed, daily oral contraception is safe and is expected to be more effective than currently available nonprescription contraceptive methods in preventing unintended pregnancy.”

Since the Supreme Court overturned the national right to an abortion last year, the accessibility of contraception has become an increasingly urgent issue. But long before that, the move to make a nonprescription pill available for all ages had received widespread support from specialists in reproductive and adolescent health and groups like the American Medical Association, the American College of Obstetricians and Gynecologists and the American Academy of Family Physicians.

In a survey last year by the health care research organization KFF, more than three-quarters of women of reproductive age said they favored an over-the-counter pill, primarily because of convenience. Nearly 40 percent said they would be likely to use it. Those most likely to opt for the product included women already taking birth control pills, women without health insurance and Hispanic women, the survey found.

And strikingly, at a time of fierce divisions over abortion, many anti-abortion groups have declined to criticize over-the-counter birth control. Opposition appears to come primarily from some Catholic organizations and Students for Life Action.

In May, a panel of 17 independent scientific advisers to the F.D.A. — including obstetrician-gynecologists, adolescent medicine specialists, a breast cancer specialist and experts in consumer health behavior and health literacy — voted unanimously that the benefits of making a birth control pill available without a prescription vastly outweighed the risks.

The panel cited the long history of safety and efficacy of Opill, which was approved for prescription use 50 years ago. The over-the-counter pill will be identical to the prescription version, which is 93 percent effective at preventing pregnancy with typical use.

Several panelists said there was a pressing public health need for an over-the-counter option in a country where nearly half of all pregnancies are unintended.

“The evidence demonstrates that the benefits clearly exceed the risks,” said one advisory committee member, Kathryn Curtis, a health scientist with the Centers for Disease Control and Prevention’s division of reproductive health.

She added: “I think Opill has the potential to have a huge positive public health impact.”

For proponents of over-the-counter pills, the main issue is affordability.

“If available equitably — meaning that they are priced affordably and fully covered by insurance — over-the-counter birth control pills will be a game-changer for communities impacted by systemic health inequities,” said Dr. Daniel Grossman, director of Advancing New Standards in Reproductive Health, who has led research on over-the-counter contraception.

The Affordable Care Act requires heath insurance plans to pay for prescription contraception, but not over-the-counter methods. Some states have laws mandating coverage of over-the-counter birth control, but most states do not. The KFF survey found that 10 percent of women would not be able or willing to pay any out-of-pocket cost for contraception. About 40 percent would pay $10 or less per month, and about a third would pay $20 or less.

Under a recent executive order by President Biden, the federal government could soon take steps toward requiring insurers to cover over-the-counter birth control. And Senate Democrats have reintroduced legislation to require such coverage.

“We need to make it affordable and available,” Senator Patty Murray, a Democrat from Washington State and the lead sponsor of the bill, said in an interview in May. “Let’s provide women what they need and make sure it’s affordable so there’s equity, and women who are low-income, women who for whatever reason are struggling don’t have to be forced to not have any birth control simply because they can’t afford it today,” she added.

Opill is known as a “mini pill” because it contains only one hormone, progestin, in contrast to “combination” pills, which contain both progestin and estrogen. A company that makes a combination pill, Cadence Health, has also been in discussions with the F.D.A. about applying for over-the-counter status.

The F.D.A. analysts who evaluated the data Perrigo submitted in its application for a nonprescription Opill had raised concerns about whether women with medical conditions that should preclude them from taking birth control pills — primarily breast cancer and undiagnosed vaginal bleeding — would follow the warnings and avoid the product. The F.D.A. analysts also raised questions about whether younger adolescents and people with limited literacy could follow the directions.

But in a memo explaining the approval decision on Thursday, Karen Murry, deputy director of the F.D.A.’s office of nonprescription drugs, wrote, “For an individual consumer of the product, the risk is very low, and almost nonexistent if they read and follow the labeling.”

“Overall,” she continued, “the total public health impact of the potential harm related to incorrect use by people with progestin-sensitive cancer is likely outweighed by the probable larger public health impact of prevention of a large number of unintended pregnancies with all their attendant harms.”

Several advisory committee members said patients with breast cancer, the main medical condition that precludes taking hormonal contraception, typically have doctors who would advise them to avoid birth control pills. They also said that Opill might actually be safest for adolescents because they are very unlikely to have breast cancer. And because young people often start off with contraception they can buy over-the-counter, it is especially important for them to have easy access to a method more effective than condoms and other birth control products available in retail stores, the panelists said.

Perrigo reported that participants in a study took Opill on 92.5 percent of the days they were supposed to take it. Most participants who missed a pill reported that they had followed the label’s directions to take mitigating steps, such as abstaining from sex or using a condom, Dr. Stephanie Sober, the company’s U.S. medical liaison, said at the advisory committee hearing. She said that among 955 participants, only six became pregnant while using Opill.

Most people who said they had missed doses attributed that to running out of pills before they could get to one of the study’s resupply sites, results that, Dr. Sober said, “illustrate precisely the barriers to adherence that could be lessened” by making the pill available over the counter.

PABLO, Mont. — In the mountain streams of southern British Columbia and northern Montana, a rugged part of the world, fish with misshapen skulls and twisted spines have been caught over the years.

Many scientists attribute the malformed creatures and declines in certain fish populations to five enormous open-pit coal mines that interrupt this wild landscape of dense forest flush with grizzly bears and wolves.

For decades, these mines owned by Teck Resources, a multinational mining company based in Canada, have been the subject of environmental concerns because of chemicals like selenium, a mining waste product, that leach into mountain rivers flowing through Indigenous land and across the border into U.S. waterways.

Selenium is a naturally occurring chemical important in the environment as a trace element. But selenium pollution has long been recognized as an extremely hazardous byproduct of coal mining. In larger concentrations, the chemical accumulates in the eggs and reproductive organs of fish and birds, and can cause a variety of detrimental effects, including lowered reproduction, deformities and death. The risk to human health from eating contaminated fish is not well understood.

Teck has repeatedly disputed various state and federal regulatory standards over what should be considered as safe levels of selenium in waterways. And those limits differ for lakes and rivers and between countries, complicating oversight efforts.

The latest case involves Montana and Idaho, where environmentalists’ lawsuits are waging a campaign over levels set by Montana for Lake Koocanusa in 2020. Its state standard is being challenged as a debate rages over cross-border pollution of the waterways, part of the war among regulators, tribal nations and scientists against Teck over whether the levels pose a hazard to aquatic life.

In a letter in the journal Science in 2020, a group of scientists warned of the cross-boundary pollution from Canadian mines and criticized what they and others attributed to a lack of regulatory oversight. “Mine assessment and permitting do not require incorporation of transparent, independent and peer-reviewed science,” they wrote. In Canada, they said, “Teck’s Elk Valley permit allows contaminant discharges up to 65 times above scientifically established protective thresholds for fish.”

They urged the Canadian and U.S. governments to begin bilateral negotiations, through the International Joint Commission, although previous appeals had gone unheeded.

In March, President Biden and Prime Minister Justin Trudeau of Canada pledged to reach an agreement by this summer to reduce and mitigate the water pollution in the Elk-Kootenay watershed. U.S. and Canadian officials say they are engaged in talks to set up a bilateral process in the coming months.

On the Montana standard, Teck has challenged the state’s levels, which are more restrictive than U.S. standards. “Significant, ongoing monitoring confirms selenium concentrations in the Koocanusa Reservoir are safe, not increasing and have been stable since 2012 and do not pose a risk to aquatic or human health,” said Chris Stannell, a spokesman for Teck.

In its annual report for 2022, the company said it was continuing “to engage with U.S. regulators to work toward the establishment of appropriate science-based standards for the reservoir.” And the company has also pointed out that there are other lakes in Montana with high levels of naturally occurring selenium.

Scientists in Montana, however, do not accept Teck’s assessments or assertions of levels in the lake, which spans the international border. Selenium levels in the Kootenai River have not decreased, environmentalists said.

At the site where the Kootenai River empties into Lake Koocanusa in Canada, levels have steadily increased well above those metrics, said Erin Sexton, a senior scientist at the Yellow Bay Biological Station, operated by the University of Montana on Flathead Lake.. “It’s a hockey-stick graph, it just goes up and up,” she said.

Provincial government officials say there are “robust monitoring and assessment programs in place” that have not detected any effects.

The environmentalists’ lawsuit is seeking to preserve the more restrictive Montana standard, which has come under fire from Republican-led lawmakers and some state agency officials. In its company report, Teck questioned whether the lower limit was in force, presumably because of the internecine feud among state authorities.

The U.S. level for selenium in rivers is 1.5 micrograms per liter in lakes and 3.1 in flowing rivers. Montana’s standard for Lake Koocanusa, after six years of research, was set at 0.8. The level for protection of aquatic life in British Columbia is 2.

Tracing the waterway chemicals

When rain falls or snow melts, waste rock from the mines leaches selenium into waterways. The level of selenium in the Fording and Elk Rivers in British Columbia near the mines have at times reached levels many times higher than provincial standards. A population of genetically pure cutthroat trout in the Fording River was decimated, at least in part, by selenium levels. In 2021, Teck was fined $60 million, a record, under Canada’s Fisheries Act for release of selenium into the Fording River.

The Elk River flows 140 miles from its source until it enters Lake Koocanusa, created by the damming of the Kootenai River, which straddles the border. The lake becomes the Kootenai River again below the Libby Dam in Montana and Idaho, and it flows back north into Kootenai Lake in British Columbia. Eventually it empties into the Columbia River.

Unlike in an oil spill, the effects of high selenium levels do not result in large kills of fish that suddenly appear belly up in the water. Instead, selenium poisoning reduces fish numbers by causing mortality in the larval stage.

“It’s a really nasty contaminant because it causes deformities in reproductive organs,” Ms. Sexton said. “They call it an invisible contaminant because they fail to thrive. You don’t find eggs that don’t hatch.”

Indigenous land concerns

U.S. and tribal officials argue that the mining-related presence of chemical pollution violates the 1909 International Boundary Waters Treaty. Tribal leaders in the United States claim it may breach their treaty rights of 1855, which guarantee “taking fish at all usual and accustomed places.” They want the International Joint Commission, a U.S.-Canadian body that oversees trans-boundary disputes, to study the issue and prepare recommendations for cleanup.

But the government of British Columbia has continued to oppose efforts to remedy the situation, Montana officials say.

Part of the problem, Ms. Sexton said, is that the province of British Columbia allows industries, including mining companies, to largely monitor themselves and offer evidence produced by their own scientists.

Mr. Stannell pointed out that the company had spent $1.2 billion on wastewater treatment near the mines, and planned to spend $750 million more to improve water quality in the coming years.

A region rich in coal

Coal mining in this Canadian province began more than a century ago, though the methods changed in the 1980s, when underground mining was largely abandoned for open pits.

High-grade metallurgical coal is mined using a method called cross-valley fill, similar to the mountaintop-removal technique employed in West Virginia and some other states. Explosive charges blast away the top of the mountains, removing whole sections of a range, to expose rich deposits of coal. Giant shovels and massive 550-ton dump trucks mine the coal, which is transported by rail to Vancouver and then by ship to Asia, where it is essential for steel manufacturing.

Other mining-related pollutants, partly from the extensive use of explosives in the blasting, include cadmium, sulfates and nitrates, experts say.

Teck’s open-pit mines produce more than 21 million metric tons of coal a year. An analysis issued last year by the British Columbia Chamber of Commerce estimated that the company provided nearly 13,000 jobs in the province and $4.6 billion to its gross national product.

The company is seeking to expand one of its five Elk Valley mines, and it has applied to open two new ones.

Calvin Sandborn, the legal director of the Environmental Law Center at the University of Victoria and one of the authors of a 2021 report, accused the governments of British Columbia and Canada of deliberately failing to regulate Teck.

“If they had acted on the warnings of their scientists years ago, they would have dealt with this problem,” Mr. Sandborn said. “And they didn’t because it’s a corporation that’s too big to fail.”

According to the Elk Valley Water Quality Plan, British Columbia allows Teck to continue to operate its mines as long as it stabilizes selenium levels and eventually reduces them after 2030.

Scientists worry that the existing mines could pollute the rivers for centuries. And some do not believe the technology exists to remove enough selenium from flowing rivers or groundwater to reach safe levels. Ms. Sexton said Teck could do more to seal the contaminants in the waste rock.

Critics of government policies point out that when John Horgan stepped down from his post as premier of British Columbia in 2022, he became a member of the board of Elk Valley Resources, a spinoff of Teck Coal, created to manage the mining resources. According to BIV, a publication that covers business in British Columbia, board members are paid at least $235,000 annually.

The Canadian Ministry of the Environment and Climate Change Strategy did not respond directly to questions seeking a response to accusations that the government’s oversight of the mining giant was inadequate or lax.

“We are pleased that Canada and the United States have committed to working together to reduce and mitigate the impacts of water quality concerns,” said David Karn, a spokesman for the agency. “Protecting and enhancing water quality is a key priority, and through our regulatory activities, we continue to undertake and oversee projects to improve and protect water quality in the Elk River Valley and Koocanusa Reservoir.”

New policies cannot come soon enough for Indigenous people and conservationists in both countries.

The mining on the ancestral lands of the Kootenai people, (known as Ktunaxa in British Columbia), has become a longstanding issue. “Over a century of mountaintop-removal mining has laid waste to the traditional territory of the Ktunaxa Nation, contaminating the Kootenay River and fish that depend on it,” the six governments of the tribal nation said.

“Our native fishery is extremely important to us,” said Tom McDonald, chairman of the Confederated Salish and Kootenai Tribes at their headquarters in Montana and a fisheries biologist. “Water to us is almost considered holy water. It’s very sacred.”

“You catch a fish and its gill plate is missing or its jaw is all malformed, are you going to eat it?” Mr. McDonald said. “No, you are not going to. When you lose that ability to fish, it disconnects you from your culture. It takes a whole thing away from the people — their society, their sense of place, their community and their family. It’s an extreme taking.”

The Kootenai/Ktunaxa tribes have worked to protect water quality and fisheries in their territory. The Kootenai band in Bonners Ferry, Idaho, has a long-term program to restore burbot to the Kootenai River. The long, eel-like fish, known for its flaky white flesh, is important for subsistence fishing, and it almost became extinct before the tribe built a hatchery to rear fish for introduction back into the river. Now, selenium has been found in the fish there.

Whitefish populations below Libby Dam, which created Lake Koocanusa, have declined considerably in recent years. Monitoring in 2018 found that populations, which are usually 700 fish per 1,000 feet, were down 53 percent in 2018 and 55 percent in 2023. High levels of selenium, above state and U.S. limits, have been found in fish eggs and ovaries.

The selenium from the mines “is likely causing the decline,” said Jim Dunnigan, a fisheries biologist for the Montana Department of Fish, Wildlife and Parks who is studying the contamination. “It’s cause for serious concern.”

Wyatt Petryshen of Wildsight, the Canadian environmental group that monitors Teck’s operations, said environmentalists were worried about recent moves by Teck that split its operations into Teck Metals Corp. and Elk Valley Resources, which will own the coal-mining operation.

“There are very real concerns that Teck is trying to spin off the company to avoid paying for environmental damages while maintaining cash flow to their metal mining business,” Mr. Petryshen said.

Sheila Murray, chairwoman of Teck’s board, defended the change, saying it would be more profitable for shareholders and would “support a sustainable future for the benefit of employees, local communities and Indigenous peoples.”

U.S. officials and advocates said the International Joint Commission, the bilateral body, would be the best authority to seek ways to contain and reduce the mining pollutants. “We need a scientific advisory panel of both U.S. and Canadian scientists,” Mr. Sandborn said. “We need to get this to the International Joint Commission so that we have a proper watchdog.”

Jennifer Savage, a spokeswoman for the U.S. State Department, which oversees the United States’ role in the commission, said it wanted the international body to take the matter up soon.

“Indigenous communities along the watershed depend on these waters for cultural survival and for their survival,” said Ms. Savage, director of the department’s office of Canadian affairs. “We’re impatient. We are definitely eager to find a solution.”