A single-use, clamshell protective cap for use primarily in healthcare settings in order to maintain the integrity of a medication solution. The cap has two halves, a cylindrical skirt, a puncture resistant lid portion, an interior flange, and an interlocking snap. When placed around a flanged injection port such as is found on a conventional fluid container and closed, the lid portion covers an access site on an injection port. After the cap is placed, the interior flange engages the port flange to prevent upward axial movement relative to the port, thereby preventing removal of the cap and thus deterring unwanted or erroneous drug administrations or withdrawals.