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The United Kingdom is considering limiting the over-the-counter sale of spot-on flea and tick treatments for pets due to environmental concerns.

The United Kingdom government is considering confining the sale of certain flea and tick treatments to veterinarians and pharmacists, citing what it says is growing evidence that the products are contaminating the environment, particularly waterways.

Many popular brands of flea and tick treatment for dogs and cats are available over the counter in the U.K. at supermarkets and pet stores. Although the government didn't specify what types of treatment would be affected, it has funded research over recent years focused on topical products, also known as spot-ons, that contain the insecticidal chemicals fipronil and imidacloprid.

The government didn't state explicitly whether a prescription from a veterinarian could be mandated under any new rules. It would only say that sales may be restricted to veterinarians, pharmacists and "suitably qualified persons." The latter are defined in the U.K. as anyone trained adequately to prescribe and supply prescription-only veterinary medicines.

"These treatments play a vital role in pet health and welfare, but it is right that we look at whether they should only be made available for sale via medical practitioners who can advise the public on their correct usage," the U.K.'s water minister, Emma Hardy, said in a press release.

Aimee Carguillo, a spokesperson for the U.K.'s Department for Environment, Food and Rural Affairs, indicated that a prescription may not necessarily be required if products are purchased at pharmacies, including online pharmacies. "If supply was restricted following this review, these medicines would still be available online, but only via registered retailers that can provide professional advice upon dispensing, such as online pharmacies," she said in an email.

The government has launched a "call for evidence," with submissions due June 11. The review is expected to take 12 months and, if any changes are decided, businesses would be given "sufficient time" to implement new rules, the government said.

The U.K. appears to be among the first, if not the first, country in the world to consider placing restrictions on the over-the-counter sale of spot-ons. Concerns about their environmental impact rose in 2021, when a study funded by the U.K. government's Veterinary Medicines Directorate (VMD) identified high concentrations of fipronil and imidacloprid in thousands of samples taken from 20 English rivers. The chemicals, apart from killing disease-causing fleas and ticks, also can kill tiny aquatic insects that are a crucial food source for fish and birds. That research and various other studies — including some conducted in the United States, such as this 2017 study — hypothesize that the treatments contaminate waterways after being washed down drains when pets are bathed, owners clean their hands or pets swim in rivers.

Spot-ons likely are a large contributor to environmental contamination, the British research suggests, because the use of fipronil and imidacloprid for agricultural purposes has been banned in the U.K. for close to a decade. The chemicals still are permitted for use in household pest control and for ornamental plants. A subsequent VMD-funded study published in 2024 found that owners bathing pets, washing their beds and cleaning their hands caused significant fipronil and imidacloprid contamination in U.K. waterways.

The U.K. government said in Thursday's press release that the country's Environment Agency has found concentrations in surface water that frequently exceed toxicity thresholds for aquatic insects. Residues, it adds, also have been found in river sediments, fish tissue, wild bird nests and coastal waters.

A paper published in February 2025 found fipronil, imidacloprid and another insecticidal chemical, permethrin, in the nests of two bird species: blue tits and great tits. Birds often line their nests with animal fur, providing a pathway for contamination from spot-ons. The researchers found a higher number of either dead offspring or unhatched eggs in nests containing higher concentrations of insecticides.

Fipronil and imidacloprid are active ingredients in many popular spot-on brands, such as Advantage and Frontline Plus. The National Office of Animal Health (NOAH), which represents veterinary pharmaceutical companies in the U.K., maintains there is no firm evidence nor scientific consensus that spot-ons are harming the environment. The origin of chemicals in waterways could come from multiple sources, it argues, such as pest control products like cockroach and ant baits and the residual presence of chemicals from historical agricultural use.

In its response to the U.K. government's rule review, NOAH highlighted the importance of flea and tick treatments for controlling diseases that affect both pets and humans.

"Parasite control is a cornerstone of responsible pet ownership," its chief executive, Dawn Howard, said in a press release. "While we support a thorough and transparent review, it is essential that decisions are grounded in sound science and a full understanding of the real-world impacts on animal health and welfare."

Howard continued: "Pet owners are already under financial pressure, and affordability directly influences whether animals receive the treatments they need. Any move that increases costs or reduces access risks leaving more animals unprotected."