A vented vial adapter for reconstitution of a medicament in a vial includes a vent lumen and a medicament lumen. The vent lumen has a cross-sectional area equal to or greater than that of the medicament lumen so that the fluid flow rate through the vent lumen is equal to or greater than that of the medicament lumen. As a result aerosols of the medicament outside the adapter are avoided as is the withdrawal of air bubbles from the vial as the reconstituted medicament is withdrawn from the vial. To maintain the diameter of the sharpened cannula of the vial adapter as small as possible, the vent lumen includes a concave wall facing the medicament lumen and a convex outer wall. In one embodiment, the vent lumen is in the shape of a polygon.
