I have't read about any warnings other then posts from a few people on quilting boards. Mosst haven't used the pens or researched the pens to know anything about them. Lots of professional show quilters blog about these pens and how great they are. If fabric can handle the harsh chemicals used in manufacturing, it can hold up to a non toxic marking pen. The whitish marks left on dark fabric do go away. At least the ones on my quilt did.