You may want to consult your vet before trying these. No, they’re not medicine, but I am not sure about the claims made by the makers about the oral hygiene claims made. I am guessing different vets will have different opinions here.
These are elongated chewy treats, made of freeze-dried fish with some additives (including antioxidants, which are supposed to be a good thing). They also contain two types of enzymes, that are supposed to prevent plaque from forming, and generally clean teeth.
If your cat needs periodic teeth cleaning, this is probably not a replacement (actually the makers, CET say as much about their product), but it can make a nice teeth-friendly treat every now and again. These treats are also available in chicken flavor.