Common flea and tick medications for dogs and cats are shedding active chemical compounds into the environment at levels that could pose a significant risk to insects vital to ecosystem health, according to a study published January 14, 2026, in Environmental Toxicology and Chemistry.
The research, conducted by French scientists and published by Oxford University Press on behalf of the Society of Environmental Toxicology and Chemistry, focused on isoxazoline drugs – a relatively new class of antiparasitic medications introduced in 2013. These drugs, including fluralaner, afoxolaner, lotilaner and sarolaner, are widely prescribed by veterinarians for monthly or longer-term flea and tick control in companion animals.
The study examined the fecal elimination of these drugs in 20 dogs and 20 cats owned by veterinary students over a three-month period. Researchers found that even after the recommended treatment period concluded, detectable levels of fluralaner and lotilaner remained in the animals’ feces. Specifically, median half-lives were estimated at 15.5 and 22.0 days for fluralaner and lotilaner in cats, and 22.9, 24.6, 19.7, and 17.4 days for fluralaner, lotilaner, afoxolaner, and sarolaner in dogs, respectively.
The concern stems from the way these drugs are metabolized and excreted by pets. Isoxazolines are primarily eliminated via the biliary/fecal route, meaning the active compounds pass through the digestive system and are released into the environment through waste. Researchers suggest that these compounds can also enter ecosystems through urine and shed pet hair.
The European Medicines Agency has previously highlighted the potential for these substances to contaminate ecosystems, but detailed data on the extent of environmental release has been limited. The study’s findings indicate that dung-feeding insects – including flies, dung beetles, and certain butterflies – are particularly vulnerable. These insects play a crucial role in nutrient cycling, soil health, and pest control, and their consumption of feces containing isoxazoline residues could disrupt these vital ecological processes.
An environmental risk assessment conducted as part of the study suggests that dung-feeding insects could experience high levels of exposure to isoxazoline compounds. The researchers determined that fluralaner and lotilaner pose the greatest potential impact. Monte Carlo simulations were used to assess the risk to non-target arthropods.
The study builds on earlier research indicating that isoxazoline antiparasitic drugs exhibit long plasma half-lives in both dogs and cats, with lotilaner reaching 30 days and sarolaner up to 41.5 days in cats. Absorption of lotilaner can also be significantly reduced by fasting.
The Society of Environmental Toxicology and Chemistry (SETAC) highlighted the research in a statement released January 29, 2026, emphasizing the demand for further investigation into the environmental consequences of widespread veterinary drug use. No immediate regulatory response has been announced.