You can find SCP-500 inside small glass cases inside Light Containment and Heavy Containment - it will resemble a small pill bottle, not to be confused with the normal pill bottles found in lockers. You will need a keycard with Containment Access Level 2 and Checkpoint Access. There are multiple instances of SCP-500 in every round.
A simple passive way to get this achievement is to drink SCP-207 (the soda) or to take fall damage until you are at about 10HP. Using SCP-500 will completely heal you and cure any ailment, granting you the achievement.