To suck on s.t. (as candy, medicine, a mother's breast).
When -an imp. is used, it focuses on s.t. being removed by sucking it from s.t. else (like poison from a snake bite). When -un is used, it focuses on the object being sucked and the contents of the object are retained in one's body.
If you are fond of sucking on candy, after awhile your teeth will hurt.