Walk into any pet store and you’re likely to see shelves of topical products aimed at killing and repelling fleas and ticks on dogs and cats from brands such as Frontline, Advantage, and Seresto. Almost all of them contain the pesticides fipronil or imidacloprid. They are, as you might expect, toxic to invertebrates.

But they don’t stay on your furry companions. 

When you wash your dog or its bedding, or pet your cat and wash your hands, the chemicals go down the drain, where they head to a wastewater treatment plant—a facility that was not designed to remove pesticides.

Studies show that fipronil and imidacloprid are ubiquitous in Bay Area wastewater effluent at levels that the EPA has deemed may be harmful to aquatic life. That means they end up in the Bay, where, scientists say, they may pose risks to invertebrates such as shrimp, amphipods, and isopods—bottom-dwelling creatures that provide important food for fish, birds, and marine mammals.

The data is worrying enough that California’s Department of Pesticide Regulation is currently considering limiting the use of fipronil. (Fipronil is also classified as a possible human carcinogen based on animal studies.) DPR plans to release possible mitigation options for public comment before October 1. 

If the chemical is restricted, it could force millions of California pet owners to change how they keep their pets safe from fleas and ticks.

Solving a chemical mystery

Nearly two decades have passed since DPR found that fipronil and imidacloprid were common in the state’s surface waters. The question was how they got there. 

In California, these pesticides aren’t just applied to pets—they’re used in residential, commercial, and (in the case of imidacloprid) agricultural settings. Fipronil was first registered for use in California in 1997 to kill ants, termites, and cockroaches. Imidacloprid, registered inthe state since 1994, is a neonicotinoid, invented to mimic nicotine. Farmers use it to control aphids, beetles, and grubs on grapevines, vegetables, and other crops, and anyone can use it to kill ants and cockroaches in their homes. Both chemicals work by disrupting the central nervous systems of insects and are highly toxic to aquatic life, especially invertebrates. Fipronil is also highly toxic to marine and freshwater fish, while imidacloprid is slightly toxic to fish (although it depends on the species). 

In urban settings, outdoor structural use was a possible transmission route, but so was wastewater. In September 2015, the San Francisco Estuary Institute, in collaboration with the California Department of Pesticide Regulation (DPR), went looking for fipronil and imidacloprid in the influent and effluent of eight wastewater treatment plants in the Bay Area—and found their traces in 100 percent of the samples. The minimum concentrations of fipronil all exceeded the EPA’s safe threshold for aquatic species.

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“We knew at the time that fipronil and imidacloprid and some pyrethroids were slipping through these sewage treatment processes,” says Stephanie Hughes, a chemical engineer who works for the City of Palo Alto and the Bay Area Clean Water Agencies (BACWA). “So then it was the puzzle, the challenge to figure out: where are these coming from?” 

It was clear, at least, that the source was coming from someplace indoors, since most urban runoff in the Bay Area goes straight into local creeks and the Bay without hitting a wastewater treatment plant first. 

Another DPR study found that wastewater from pet groomers had very high concentrations of fipronil, imidacloprid, and permethrin (another pesticide used in pet products) compared toother possible sources, such as laundromats (which might contain clothing or fibers exposed to pesticides used in homes) and pest control operators. That made spot-on anti-flea-and-tick topicals used on dogs and cats—that’s a few drops of liquid often placed on the back of the neck—the prime suspects. 

DPR decided to focus on fipronil because, while it is also used to kill pests around the edges of buildings and inside homes, it is one of the most common ingredients in pet products, almost all of which are topical spot-on treatments. (There are no approved uses of fipronil in agriculture in California.) Imidacloprid, on the other hand, is found in hundreds of products used for agriculture, structural pest control, home gardens, and pet products. “With so few potential sources, fipronil found in wastewater can be more clearly linked to pet treatments,” J.T. Teerlink, DPR’s deputy director and chief science advisor, explained in an email.

To confirm that pet topical products were the source of fipronil found in wastewater, DPR conducted a study to see how much fipronil would wash off a dog during bathing and end up down the drain and in wastewater. Even though many of the products claim to be waterproof after 24 or 48 hours, they found fipronil and its degraded forms in the rinse water up to 28 days after application. “If 20 percent of the dogs that had fipronil applied were washed at 7 days [after application] and it went down the drain, that would represent the total amount of fipronil that we’re seeing in wastewater,” says Teerlink, who coauthored the study. 

Nearly a decade after that study was published, fipronil and imidacloprid are still widespread in wastewater. Monitoring of 28 wastewater treatment plants in nine counties throughout California in 2024 found fipronil and imidacloprid in nearly all the effluent samples they tested—most at concentrations exceeding the EPA’s lowest safe threshold for freshwater invertebrates. 

That safe threshold is just 11 nanograms per liter for fipronil and 10 ng/L for imidacloprid. For reference, 1 ng/L is equivalent to one drop in 20 Olympic-sized swimming pools, reflecting how sensitive aquatic organisms are to very small concentrations.

Chemicals found at or above toxic thresholds 

What harm do these chemicals do once they get into the Bay? It depends on how diluted they get. If they are in effluent that is discharged into smaller streams, they are more likely to cause ecological damage than if they are discharged into larger rivers. Up to a third of the streams that receive wastewater in California lack the flow to dilute chemicals meaningfully. And 16 bodies of water in the state are already considered impaired due to fipronil or a toxic degradate, including Castro Valley Creek in Alameda County, Nathanson Creek subwatershed in Sonoma County, and Petaluma River in Marin and Sonoma Counties.

In San Francisco Bay, concentrations of fipronil are “approaching aquatic toxicity thresholds,” says Miller, while levels of fipronil sulfone (a degradate of fipronil) exceed the EPA’s aquatic life benchmark. Past monitoring of fipronil and fipronil sulfone in Bay sediment found concentrations that exceeded a sediment-specific toxicity threshold developed at UC Davis. 

While DPR is continuing to monitor wastewater for fiproles and imidacloprid, the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP)—a collaborative of regulators, wastewater agencies, industry, nonprofits, and scientists—is not prioritizing more monitoring at this time. The RMP has listed these pesticides as a moderate concern, the same tier as bisphenols (such as BPA) and microplastics. But another organization, the Bay Area Pollution Prevention Group, ranks fipronil and imidacloprid at the highest level of concern. 

Pesticides in the Bay exist in a stew of lots of other chemicals, and it’s very hard to quantify any one chemical’s specific effects. Still, says Ezra Miller, senior scientist at the San Francisco Estuary Institute, “there is the possibility that these concentrations could cause some kind of harm, and in the long run, that could cause ecosystem-level problems.”  

Bay Nature staff pup Mochi cools off on a hot day in Lotus, east of Sacramento; she gets monthly oral medications to ward off fleas and ticks. Sixteen water bodies across California are listed as impaired for fipronil or a toxic degradate, but for all the waters outside the Bay, the source of the contamination is listed as “unknown.” Kate Golden

Since the pandemic, when isolated people working from home led to a steep increase in pet ownership, use of these pesticides has risen dramatically. About two-thirds of U.S. households now own a pet. Between 2013 and 2019, 16,600 pounds of fipronil were sold in pet products each year in California. That number jumped to 41,600 pounds between 2020 and 2022. According to the California Veterinary Medical Association, vets are recommending products with fipronil less often, but pet owners who can’t afford basic veterinary care (estimated at approximately one in four pet-owning households) still rely on them. As of June 23, 2026, there were 97 total products containing fipronil actively registered in California, including 57 pet products, according to DPR data.

The state is mulling fipronil risks

Pesticides don’t just threaten aquatic organisms. In 2023, DPR assessed the potential risks to people from fipronil  in part because there were so many human health effects adverse reports—over 6,000. Acute symptoms included vomiting, diarrhea, vertigo, and other problems. Out of the approximately 6,800 reports, two people died. Roughly 80 percent of the adverse reports were associated with pet products. 

The combination of environmental and human health risk led DPR to draft an aquatic risk memo for fipronil, which was finalized at the end of July. It summarizes the wastewater monitoring data to date and assesses the risks to surface waters. Next, DPR plans to come up with options to address the risks, including an analysis of alternatives.

Miller hopes a restriction on fipronil won’t lead to a “regrettable substitution”—like people just shifting to using imidacloprid or pyrethroids instead. In a December public comment on the draft aquatic risk memo, Lorien Fono, the executive director of BACWA, notes, “Imidacloprid in treated effluent is 6.9 times greater than that [aquatic life benchmark] while fipronil is 2 times greater than its respective ALB, suggesting that imidacloprid toxicity may be more of a threat to … receiving water than fipronil.”

Meanwhile, the U.K. is currently considering a ban on fipronil and imidacloprid in over-the-counter products. In an August 2026 report, the House of Lords’ Environment and Climate Change Committee urged the government to end sales of over-the-counter pet pesticides, citing the lack of “convincing evidence of clinical need” and noting “enough evidence of environmental harm” created by pet pesticides.

What should pet owners do? Opinions vary.

Jane Sykes, a professor of small animal internal medicine at UC Davis’ Weill School of Veterinary Medicine, does not recommend a complete ban on these pesticides, because she says they’re an important public health tool for fighting problems like flea- and tickborne infectious diseases and roundworm infestations in children. Instead, she says the pet industry should improve product labeling to include language about the potential for environmental contamination, especially for animals that are bathed or swim frequently. 

Regulators focused their investigation on spot-on treatments, but these chemicals are also sold in sprays. The Advantage sprays here contain imidacloprid, which has been found in Bay waters. Amir Aziz/Bay Nature

There are alternatives to spot-on topicals, such as oral and chewable products. These usually require a prescription and are more expensive than over-the-counter options, making them less accessible. And they may not be benign, either. A study earlier this year in France found that the pesticides in oral treatments are excreted through pet feces, urine, and hair, and may pose risksto non-target insects such as flies, beetles, and butterflies. (Ingestibles are regulated by the U.S. Food and Drug Administration, not the state DPR.)

The Companion Animal Parasite Council—a nonprofit funded by major manufacturers of pet pesticides that includes veterinarians and researchers on its board—currently recommends that all dogs and cats be treated year-round and throughout their life with flea and tick control products. But Sykes says this recommendation is excessive and unnecessary. Instead, she thinks the pet industry should develop tailored products “that are specific for certain lifestyles,” such as for dogs who swim frequently. 

For now, the trend appears to be going in the opposite direction: Pet product manufacturers have been developing broad-spectrum combination products that can be conveniently given in one dose to last a whole year to target a range of parasites. “That sort of lack of discriminate use has the potential to contribute to these concerns about environmental contamination, effects on non-target invertebrate hosts, and also, the development of resistance,” says Sykes. “Let’s figure out what parasites are a problem in this area and then prevent those.”

Hughes and Sykes have been busy trying to educate veterinarians and product manufacturers about the issue at meetings and conferences. “There are veterinarians going, ‘Wow, I had no idea,’ and even people who work for industry groups actually are slowly becoming more aware,” says Sykes.

For now, the responsibility to lessen these pesticides’ impact on the environment lies on pet owners.

On a sunny and breezy Monday afternoon in July, dozens of people were walking their dogs at Point Isabel Regional Shoreline in Richmond, one of the few East Bay parks that allowsoff-leash dogs, including in the Bay. Most people interviewed said they gave their dogs oral flea and tick treatments via a prescription. But Steve Work of San Pablo said he gives Mini, his eight-year-old Chihuahua mix, monthly topical treatments that he buys from Amazon or Costco. He didn’t know they contained pesticides that could end up in wastewater and the Bay. “That’s news to me,” he said. 

Julia Velson of San Francisco was walking her 7-year-old Chihuahua/Poodle/Yorkie/Shitzu/Pekinese/Pomeranian/Chow-mix Izzy, and friends Luna and Chance. While she gives Izzy oral flea and tick treatments, she occasionally uses topicals for her cats, which stay indoors. “It pains me that I give my dog a pesticide, but the consequence of not giving it to her…” she said. 

It’s a dilemma for pet owners who are trying to be responsible to both their pets and the environment. “People assume that if they can buy an over-the-counter thing that it’s safe, which I think is a very reasonable assumption,” says Miller. “And that’s not the reality, unfortunately.”