A dispenser includes a hollow containment body with a hole located inside the bottle and an orifice for pumping a liquid product contained inside the bottle. The dispenser also includes a ring nut, a piston, and a hollow stem slidable inside the body and associated with a dispensing spout to actuate the piston in order to dispense fluid contained in the bottle. The body further includes an aperture to selectively place an ambient air inlet passage in fluid communication with the hole in the body. The ring nut has a first substantially cylindrical portion and a second substantially cylindrical portion which are mutually coupled to define the air inlet passage between them.