I just asked the same question in the monitor forum. studies have shown that pine can cause cancers in some small mammals and reptiles. Now whether that is something in the actual wood of the tree or the bark I don't know. I know people who have successfully kept monitors on it for years with no problem, but most people would advise against it. I would say don't use it just to be safe. I usually just get the cypress mulch from home depot. Its just as cheap and works fine.