Yes, it's safe. It's catnip.

But the withdrawal can be somnific.

It's catnip, or another member of the Lamiaceae family. Cats are addicted to it.

Not sure the words "addicted" or "withdrawal" apply and Tim, I could see how that might concern you if you are unfamiliar with the plant. Plants in the Nepeta genus contain a chemical called nepetalactone which cats react to in various ways, or not. It is not harmful to them. Some sniff and roll in it, some eat it. Here are a couple of articles that explain it.

https://www.humanesociety.org/...

https://pets.webmd.com/cats/ca...

I had a cat once that reacted to it by turning into a whirling dervish for about an hour and then passed out for 4 more. While out, she had no visible response to being picked up, petted or anything else. A little scary but her breathing was steady so we didn't panic. Happened every time.