Thank you for your question. I have always heard that you shouldn't use cat litter for ferrets, but wanted to get the opinion from our in-house veterinarians. I spoke with them and this is the information they gave me.
We usually don’t recommend kitty litter made out of corn cobs for ferrets, since it can sometimes have mold in it. Clay litter could cause intestinal blockage if the ferret would eat it. We like to use pellets made out of recycled newspaper, such as Marshall Premium Ferret Litter. Kitty litter pellets made out of recycled newspaper would be okay, too. We prefer the unscented variety, if you can find it.
Hope this helps!