From Exterminatus Manual
Apothecaries are marines specialy trained to tend to battle wounds and the biological enchancements of his chapter. In Exterminatus they fulfill a support role, healing injured marines.
The primary tool of the Apothecary is the Narcathium, which is used to treat battle wounds and secure the chapter's geneseed from fallen marines. This is backed up with a chainsword for amputations and a bolt pistol for putting respect into xenos and heretics.
Stay close to your battle brothers, so you can keep them healed as healing earns apothecaries reward points. If you find yourself alone, fall back or hide! While the narcathium is your active weapon it will heal nearby marines and refill with meds when idle.