Hello! I hope I’m not responding too late. Zylkene is market for dogs and cats, not rabbits. It contains an ingredient called alpha-casozepine which is derived from a milk protein called casein. Rabbits are lactose intolerant and can’t digest dairy products.
I’m not a vet but I highly suggest you don’t give that to your rabbit. If your vet had to do research and this is all they found then they’re either not a rabbit-savvy/exotic animal vet or, if they are, they’re not a very good one.
There are natural ways to calm rabbits, such as herbs like chamomile. Any medicine or supplement that is not marketed to rabbits is usually an indication that it wasn’t tested on rabbits and probably isn’t wise to use.
I suggest you look into the root of why your rabbit is so nervous. Is there something in their environment that is making them feel uneasy or threatened? Here are two sources that may help: https://bunnylady.com/5-steps-to-calm-a-rabbit/
Healthy Herbs Your Rabbit Can Eat (and their medicinal properties)
This website in general is packed full of rabbit info that only has a rabbit’s best interest in mind.
Good luck, hope this helps!