You're speaking with Dr. Michael Salkin. Welcome to JustAnswer. I'm currently typing up my reply.
Laura, can you clarify "bladder constructed" for me? Cystophan contains N-acetyl D-glucosamine, hyaluronic acid, L-tryptophan and an artificial chicken flavouring. The trytophan can't relax a bladder to the extent you've mentioned. It doesn't have that mechanism of action. Urinary incontinence is usually associated with urinary tract infection in the cat. Neurogenic incontinence - often due to injury to the spinal cord - does occur but that wouldn't be a consideration in a cat with no history of trauma. Congenital (present at birth) incontinence as seen with Manx or Manx-like cats is well-known.
To answer you directly, yes cats can take human glucosamine. There are feline products containing glucosamine as well. I don't believe that glucosamine is going to be the answer to her incontinence, however. We need to clarify if her incontinence is due to surgery performed as a kitten ("bladder constructed") and bacterial infections secondary to that surgery. If so, antibiotics may be necessary indefinitely - pulse therapy (treat for a week, off a week, treat for a week, etc.) might be considered.
Thank you for the additional information, Laura. That's an very interesting history. She's very lucky to have had you.
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I appreciate it! That's all that counts!
Thank you for your kind accept. I appreciate it.