Medical device for in situ liquid drug reconstitution in medicinal vessels containing drug contents. The medical device includes a body member having a vessel port for insertion into a medicinal vessel containing drug contents, and a syringe port for receiving a syringe containing a diluent for reconstituting the drug contents into a reconstituted liquid drug. The medical device includes a one-way flow restriction mechanism between the two ports for positively restricting injection of diluent into a medicinal vessel and only slightly restricting aspiration of reconstituted liquid drug therefrom, if at all.