Willowbark is a fairly common medicine. It is used for aches.
Tam and Rand keep some willowbark among their bags of medicines. Mat, who keeps willowbark in his saddlebags, offers willowbark tea to Perrin after he speaks to the wolves. Nynaeve tells Thom to ask Rendra for willowbark or acem to relieve his bruised head. Nynaeve used willowbark after channeling too much at Algarin Pendaloan's manor.
Willow bark has been used for thousands of years and contains a chemical similar to aspirin.