An adaptor for coupling with a vial having a collar closed by a septum, the septum having an outer surface directed towards the outside of the vial, the adaptor comprising: a gripping member for securing the adaptor to the vial, the gripping member being capable of being laterally mounted on the collar of the vial and a pierceable elastomeric piece having at least a part intended to be in contact with the outer surface of the septum when the adaptor is secured on the vial. Also, an assembly including such an adaptor and a vial.