There has been quite a bit of research in the past few years and it has shown that pine and cedar shavings cause liver cancer in almost all small animals, from rabbits to gerbils and hamster and, I would imagine, rats and mice as well.....

Something in the urine mixes with a compound in soft woods and gives off an odorous gas which causes the cancer. It hasn't really been noticable in smaller animals, it is theorizied, because they don't live long to begin with - 2-3 years. But with bunnies at least, improved veterinary medicine and nutrition have extended their life span considerably. When buns started dying for no apparent reason, researchers started looking at housing, food and living conditions and found high incidence of liver cancer but could find no obvious cause. Further investigation identified the shavings as the culprit. See www.rabbit.org web site for more info.

The compound in pine and cedar (it is in all soft woods is what I have been told) is not found in Aspen or paper litters. Of course these "new" litters are more expensive than the pine and cedar shavings. The way I look at this situation is why take a chance with the shavings and that if using paper or aspen helps keep my pets healthier and have longer lives then those more expensive litters are worth it. But that's just my opinion!!!