What Does the Science Say About Medicated Grit and the Environment?

✅ KEY TAKEAWAY: Medicated grit poses little risk to terrestrial wildlife, but protecting watercourses through established best-practice guidance is essential.

A new evidence review commissioned by BASC and carried out by the Game & Wildlife Conservation Trust (GWCT) Scotland has examined every available study on the environmental effects of the anthelmintics used in medicated grit.

The headline finding is reassuring but nuanced: there is strong evidence that fenbendazole and flubendazole pose little or no threat to terrestrial wildlife or plants, but clear evidence that both compounds can be toxic in aquatic environments - particularly to freshwater invertebrates and fish.

The review concludes that existing best-practice guidance on grit placement and handling, where it is properly followed, effectively minimises the risk of water contamination.

The review also revealed a striking gap in the science. Not a single study has ever directly measured the environmental impact of medicated grit in UK upland habitats. Everything we currently know is drawn from research in other countries or agricultural settings. That makes the review's three proposals for future research all the more important.

How the review was conducted

The GWCT team carried out what is known as a Rapid Evidence Assessment - a structured, transparent method of reviewing scientific literature that sits between a traditional literature review and a full systematic review.

They searched the two largest academic databases (Web of Science and Scopus), screening over 5,000 references to identify twenty-five studies that met their inclusion criteria. Each study was then scored for quality based on features such as experimental design, replication, and statistical rigour.

The included studies spanned four continents and covered research published between 1996 and 2025. Most investigated fenbendazole (the original active ingredient in medicated grit, used until 2007) or both fenbendazole and flubendazole (the current active ingredient, marketed as Flubenvet). Only three studies looked at flubendazole alone.

What the evidence shows on land

On terrestrial environments, the picture is broadly positive. Multiple studies found no harmful effects of fenbendazole on earthworms, dung beetles, dung-colonising flies, or a free-living soil nematode. One laboratory study did report 55% mortality in earthworms exposed to fenbendazole over twelve weeks, but the concentrations used were higher than those typically found in the field, and the result contrasts with several other earthworm studies.

Research on beetles and flies is particularly relevant to moorland managers. Dung-colonising insects are an important food source for the chicks of ground-nesting birds, including red-listed wader species. The evidence suggests these invertebrate groups are not adversely affected.

Three studies examined how plants process the compounds. Harebell, common reed, and ribwort plantain were all found to take up and break down fenbendazole or flubendazole into largely inactive metabolites. However, researchers cautioned that some metabolites could revert to the active parent compound, which could theoretically contribute to anthelmintic resistance if ingested by livestock or affect free-living invertebrates.

The aquatic concern

The evidence for aquatic environments tells a different story. Water fleas (Daphnia magna) - an important indicator species found in upland peatlands - were highly sensitive to both compounds, with toxic effects observed at very low concentrations. Freshwater flatworms, worms, and amphipods showed similar vulnerability.

Studies on zebrafish embryos found that both fenbendazole and flubendazole caused developmental abnormalities, reduced survival, and organ damage at higher concentrations. One environmental risk assessment classified fenbendazole as a high risk to fish, crustaceans, and amphibians in river water.

However, there are natural factors working in favour of moorland. Both compounds are highly hydrophobic (they repel water) and bind strongly to organic matter in soil. Since peatland soils are rich in organic material, any leached anthelmintic would be expected to bind to the peat rather than wash freely into watercourses.

One study found that 70% of active flubendazole remained bound to grit kernel fat after nine months of weathering, suggesting a low leaching rate from properly administered grit.

What this means for moorland management

The review makes clear that the priority for responsible grouse moor management should be keeping the compounds out of water features. Contamination could occur if medicated grit is placed directly on the ground near watercourses, if containers are not properly secured, or through faecal excretion by treated grouse into wet areas.

Existing best-practice guidance from the GWCT and Moorland Management Best Practice already addresses these risks. Key recommendations include using small grit boxes rather than placing grit on bare ground, siting boxes at least five metres from running or open water, limiting quantities to 500g per box, and disposing of used grit through professional waste services.

A single box filled to this level contains just 0.5g of active flubendazole, of which an estimated 0.15g might leach over nine months - a small amount, especially given the compound's tendency to bind to organic soil.

Where the gaps remain

The review identifies three priorities for future research. First, a field study measuring actual flubendazole concentrations in water on grouse moors - because at present, no one has done this. Second, a laboratory analysis of flubendazole levels in red grouse droppings, since faecal excretion is a potential but unquantified route of contamination.

Third, a study of how key moorland plants such as heather and cotton grass process the compound, to understand whether plant uptake could contribute to resistance in parasitic worms or pose risks to other invertebrates.

Each of these studies would help put the management of medicated grit on a firmer evidence base, and would allow best-practice guidance to be refined if needed.

This is a careful, honest piece of work. It confirms that medicated grit, used responsibly and in line with existing guidance, presents a low environmental risk - while also making clear that the science specific to UK moorlands is overdue.

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