A cap is provided for telescoping over the puncturable seal of a medicament vial and particularly a unit dose medicament vial. The cap includes a center outwardly projecting tubular extension including outer end circumferentially spaced inwardly projecting abutments. A needle structure including a generally cylindrical body is slidingly received within the tubular extension of the cap and includes a diametrically reduced sharpened tubular needle extension on its inner end opposing and for piercing the puncturable seal of the associated vial upon movement of the needle structure inwardly of the tubular extension or mounting portion of the cap. The exterior of the tubular needle extension, spaced from the terminal end thereof, includes a bulbous enlargement for abutting against and distention of the center portion of the puncturable seal of an associated vial in order to limit penetration of the needle extension through the seal and to stretch the central seal portion to insure its penetration by the sharpened inner end of the needle structure. A cover of tubular configuration and including a closed outer end has its inner end slightly telescoped over the outer terminal end of the tubular extension and encloses those portions of the body projecting outward of the extension, the cover being inwardly displaceable along the tubular extension for engagement with the outer end portion of the needle body to inwardly displace the latter for puncturing of the associated seal by means of the needle extension.