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My name is***** and I am sorry to hear that your kitty is so lethargic. While Oleander is EXTREMELY toxic, the usual poisoning is that it slows the heart down until it stops beating and then the animal is dead. This usually happens within a few hours of ingestion.
If your kitty is still lethargic a week later, this is likely not related to the Oleander, but to some other problem. There are numerous reasons that a cat can be lethargic including fever, kidney problems, liver problems, diabetes, cancer, virus, infection and others.
Are you able to take your kitty to the vet tomorrow? They will likely recommend blood work and a urinalysis to start and maybe even x-rays.
Does this help? Do you have any other questions?
How is your kitty this evening? Is his lethargy improving? Do you have any more questions for me?