There is no doubt in my mind that the Lily is the most toxic plant to cats. Interestingly, the experts don’t know the exact chemical compound which is so devastatingly toxic to domestic cats. I think that you will find that the well-known organisation Cats Protection concurs with me on this matter.

There are three species of Lily that causes nephrotoxicosis in cats: Easter lilies, Tiger lilies, and Asiatic hybrid lilies. Other Lily species are suspected of causing nephrotoxicity. These include the day lily (Hemerocallis spp), early day lily, orange day lily, red lily, rubrum lily, stargazer lily, western lily, and wood lily (Dennis J. Chew DVM, DACVIM and Patricia A. Schenck DVM, PhD, in Canine and Feline Nephrology and Urology (Second Edition), 2011)
According to the ASPCA (American Society for the Prevention of Cruelty to Animals), there has been an increase in cases of Lily toxicosis since 2006. Science Direct states that: “The main toxin responsible for acute tubular necrosis in cats is still unknown”.

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The chemical toxin is present in the leaves and flowers. Scientists have made aqueous fractions of these parts of the flower to test on cats. I don’t know exactly how they did this and I’m not going to go into it right now. But Carrie A Palm DVM and Kayo Kanakubo DVM, in Small Animal Critical Care Medicine (Second Edition), 2015, found that cats show clinical signs of vomiting and lethargy within 12 hours of ingestion. They also showed blood biochemistry and urine abnormalities within 24 hours.

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It is suggested that cats metabolise the toxin in a different way to other species. This apparently is why lilies are so devastatingly toxic to cats.
In one study cats were “orally exposed” to these aqueous extracts of lilies (made to drink the substance or injected into their mouths?). The extracts contained the unknown poison. They say that the results indicate that the effects or symptoms of this poisoning include: vomiting, depression, polyuria, polydipsia, azotaemia, glucosuria, proteinuria, and isosthenuria as the toxic effects of the Easter lily plant.
The serum was elevated in creatinine kinase. There were significant histologic kidney changes. To put in very round layperson’s terms, lilies cause chronic kidney disease which, as I understand it, can kill a cat who has ingested lily plant material.
Why have I chosen the Lily as the client most toxic do domestic cats? It’s because you see this plant raised over and over again online and in books as to its toxicity and after reading several research papers on this it seems that the Lily is the most toxic because it has the most devastating effect in terms of health.
Note: I object strongly to the research methods.