Using hydrogen peroxide in a cat's ear is generally not recommended without veterinary guidance. While hydrogen peroxide can have some antimicrobial properties and may be used in certain situations, it can also be irritating and potentially harmful to the delicate structures of a cat's ear. Using hydrogen peroxide without veterinary guidance can lead to complications, such as irritation, inflammation, or damage to the ear canal or eardrum. It is always best to consult with a veterinarian to ensure the proper care and treatment of your cat's ears and avoid potential risks or adverse effects.