An adaptor for coupling with a medical container having a collar closed by a septum including: a tubular body closed at a distal end with a transversal wall from which extends a hollow spike, and at a proximal end by a pierceable elastomeric piece including a cavity having a plurality of circumferentially distributed chambers, each chamber being connected to the hollow spike by a radial channel, the tubular body further receiving an intermediate piece including a plurality of through holes aligned on the chambers, a selecting member having a closure wall provided with one opening, capable of rotating so that the opening is successively aligned with each of the through holes, and a gripping member for securing the adaptor to the medical container. Also, an assembly including the adaptor and a medical container.