Metronidazole (aka Flagyl) is a staple in my house, it ranks right on up there with food and litter as a necessity. 2 of my girls have IBD and need to take it on occasion so I keep it handy.
I'm not at all surprised your kitty vomited in the car after taking it. I'm guessing it went down on an empty stomach, then the car ride after being at the vets (think motion sickness on top of stress)..........I think I'd throw up, too
. Seriously, though, this drug is notorious for causing stomach upset and lack of appetite; it tastes very, very bitter & if kitty gets the slightest taste of it in her mouth, she is guaranteed to foam & salivate like nothing you've ever seen. You've got to be really, really quick & good at pilling a cat to get these down them without them ever tasting it. I give the pills, but I've been pilling mine for 7 years and have just gotten the hang of it. My first few tries pilling Gabby with Flagyl were a nightmare........and on rare occasions, if I don't get that pill just right on the back of her tongue, it'll still happen. All of that being said, it's a great antibiotic.......does the trick 9 times out of 10 with mine.
Always, always give it on a full stomach. If you've already been giving it & she doesn't want to eat, you may need to skip a dose or two until she's eating again, then re-start--only giving it when you're sure she has eaten. Also, double check with your vet about getting it compunded to one of the flavored liquids--might be easier. If, despite giving it on a full stomach, she still seems too sick or lethargic, check with your vet. It can cause lethargy, but it's always a good idea to ask if the cat seems sicker on the drugs than she was with the original problem. Good Luck...