A drug mixing system including at least one receptacle port adaptor adapted to be inserted into a port of a fluid receptacle, at least one syringe adaptor adapted to be attached to a syringe and to the at least one receptacle port adaptor and at least one vial adaptor adapted for connection to a vial containing a drug and adapted for connection to the at least one syringe adaptor, the system being characterized in that at least one of the receptacle port adaptor, the at least one syringe adaptor and the at least one vial adaptor being vented to the atmosphere in a manner which prevents release to the atmosphere of possibly harmful contents of the vial in a liquid, solid or gaseous form.