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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
I hope this is a side effect of the Xanax. In that case, the signs should wear off within 8-12 hours.
The other thing I would worry about is a saddle thrombus. This is where a clot goes to the back legs and they get hind limb paralysis. It can happen suddenly and without warning. It happens in cats with underlying, often undiagnosed, heart disease.
Unfortunately, the prognosis is poor with a saddle thrombus. Most cats are euthanized due to poor quality of life and due to the discomfort associated with it.
For now, I would give it the time for the Xanax to wear off. If still no improvement, then get an exam to determine if this is a saddle thrombus.
Best of luck. I hope everything gets back to normal. Let me know if you have any other questions.