Lactitol is a sugar alcohol, a sweetener that is derived from lactose.

I don't think I've ever seen any warning about it in relation to dairy
protein allergies. It certainly is not a problem for anybody with lactose
intolerance.

I didn't even bother putting it on my "lac" page on my web site:
http://ourworld.compuserve.com/homepages/stevecarper/experts.htm

Steve Carper