A body is provided with members for coupling it to one end of a bottle or the like. The body has a seat for housing the free end of a syringe, with a hole for allowing for the syringe needle to pass through. The free end of a syringe may be housed and threaded in the seat. On the side opposite surfaces of the body seat and at the syringe end provided for insertion in the seat, there are formed radially projecting longitudinal teeth and slanted resilient fins which are deflected by the teeth as the syringe is threaded to the body. The teeth abut against the fins, thereby preventing the syringe from being screwed off the body.