Reaction to Frontline 2 flea medicine - now what?

A little backstory - my parents have the most adorable and sweet tortie cat, she's ten years old. Very happy and content. She was mine (inherited her) but my other two cats weren't getting along with her, so we found this situation that works. She's their only pet. They let her outside occasionally but only when they're right there with her, and she's not a runner (she used to be a farm cat so the outdoors are kind of ingrained in her) The problem is, a few weeks ago I noticed she had fleas. We gave her Frontline 2 (what we give my cats) but she had a bad reaction. She had to go to the vet and get a steroid shot and he told us not to use it on her any more. She had been developing respiratory and borderline neurological symptoms. She got better, fortunately, but now she has goddamn fleas again. She can't have normal medicine because she reacts to it. Has anyone else encountered this? Any advice or anything? I'd really appreciate it.

Tldr - cat has a reaction to typical flea medicine. What do we do now?