Input.GetMouseButton NEVER returns false

Ok, so heres my code:

 if(Input.GetMouseButton(0)) {
             anim.SetBool("fireWeapon", true);
             fW = true;
         }else{
             anim.SetBool("fireWeapon", false);
             fW = false;
         }
         Debug.Log(string.Format("{0}", fW));

Initially, the debug returns false, as i havent pressed the left mouse button yet. But when I do press it it changes to true but never changes back. Veeeery confused.

Any help would be lovely ;3