The primary cause of having drooping mouth corners is the tension of muscles. The depressor angularis oris and depressor labii inferioris are the two muscles responsible for the corners of the mouth turning down. When they are overused, the repetitive action of frowning can lead to drooping mouth corners.

Another factor of drooping mouth corners is the natural ageing process, the natural decrease of collagen production leads to loss of skin elasticity. As these occur, the skin of your face can start to look sag or lax, which causes the development of drooping mouth corners. In this case, any factors that cause ageing can be the factor of having drooping mouth corner (i.e. sun damage, smoking…).