I treated all 5 of my cats with Profender after 1 confirmed case of roundworm. 4 of my cats did fine, my 5 year old female Creek had a bad reaction. She appeared fine prior to the application, but over 24 hours later she was lame, spread herself thin on the floor and started panting like mad. It appeared to have absorbed well on my other 4 cats, but left powdery residue on Creek. Took her to the vet in the morning, they said it had nothing to do with Profender. They said she had a heart murmur, did an xray and sent me to have an ultrasound. The ultrasound showed that she had infection in her gallbladder. Treated her with Clavamox, which they said probably isn't strong enough so then treated with Metronidazole. She had a terrible reaction to that, her bum was swollen, fire engine red, and she licked constantly. The vet said to leave it alone and it would clear up.
Switched vet, gave her Amoxicillin, all better now! Also relieved that she has no detectable heart murmur. Feel like I was taken advantage of by that vet. My new vet says he can't imagine why they would give Metronidazole.
It's hard to say if it was the Profender, but I have read online that pet owners have been told their cat has a heart murmur after Profender. Will probably stick to Drontal in the future for worm treatment.