My cat has open sores that won't heal. Is peroxide making it worse?

Sorry to see poor Carrie has been having these sores. It's less likely that these sores are due to an infection, and more likely that they are happening due to an autoimmune disease (self-attacking), or skin cancer. The only way to tell for sure is to have a skin biopsy done by a veterinarian and analyzed at a veterinary laboratory.

I would also want to test Carrie for FIV, which is a disease that acts similarly to HIV in cats. I've seen lesions like this in FIV positive patients in the past.

I recommend discontinuing the use of hydrogen peroxide. This antiseptic actually causes more harm to the tissue than it does the infection and is considered an outdated practice by medical practitioners. The best thing you can do for these wounds is flush them with normal tap water for five minutes twice a day. This will help keep the sores from getting infected and promote healing.

It's best to schedule a vet appointment sometime soon, because these sores will keep occurring and will get worse over time if not properly treated.

Sorry to give such scary news, but I hope this will at-least be helpful to get Carrie better.