From Exterminatus Manual
Apothecaries are marines specialy trained to tend to battle wounds and the biological enhancements 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.
Apothecaries fulfill a support role, they have enough speed to keep up with veterans, so stay close to your battle brothers and keep them healed with your Narcathium. It's generaly not worth selecting the Apothecary class unless their are at least 3 other marines in your party to support.
Healing earns apothecaries reward points. Apothecaries do not have the firepower for front line combat, if you find yourself alone, fall back or hide!
Apothecaries caught short while healing their commrades have found that the Narcathium can be used as a very effective improvised weapon against tyranids, however they are still better off using their Chainsword as it has greater reach and does more damage.
The Narcathium continuesly monitors and tunes nearby power armor, to gently heal nearby marines (including the apothecary using it) or regenerate it's reserve of meds.