I normally google first, but I thought I remembered something saying it was okay so long as I removed the strings.

It's a carcinogen. That means it causes cancer.

How bad is it, exactly? Like... should I take her to the vet now?

I don't think so. I asked my vet when I had to take care of Smosy's (my friend rat). We gave him a bit of celery but we stopped as soon as we made our research. He's still alive and fine.

That's what I suspected, but I wanted to be sure. Thanks!

I don't think a little piece will harm him. Apparently celery is a safe food for hamsters and I've owned many, but never gave it to them.

How is your rat doing?

It causes cancer, so I'll probably never know, haha. If she does get tumors within the next year, I'll have no idea if it was the celery's fault or not. Rats are prone to it even without celery.

She's fine otherwise. Pees on everything and twitches in her sleep same as always.