Apothecary
An Apothecary is a Space Marine specialist in Warhammer 40,000, combining the roles of medic and geneticist. The senior Apothecary is known as the Master of the Apothecarion (the fortress-monastery's medical center) or Chief Apothecary. They are usually armed with standard Space Marine gear and a Narthecium.
Overview
Since Space Marines have undergone genetic augmentations to survive nearly anything, an Apothecary's real battlefield roles involve three jobs: stabilization, mercy killing, and extraction. While Space Marines can endure damn near anything except the most serious injuries, Apothecaries frequently need to stabilize them long enough for their gene-seed to start up and heal them. If a Marine has been too badly injured to be saved, then the Apothecary applies "the Emperor's Peace," a mercy kill by driving a Reductor (a sharpened needle) into the Marine's skull to quickly end his pain (although some Apothecaries use bolt pistols, for some reason). If a Marine has died, it is the Apothecary's duty to extract the progenoid glands from his dead brother, to ensure that the chapter will be able to train a new space marine in the fallen brother's place.
Off the battlefield, they're basically super doctors. Their duties range from implanting initiates with the gene seed when appropriate, ensure the purity of the chapter's gene seed (or atleast make sure it doesn't get any worse), tending to recovering marines, and working on any health-related issues the chapter may task them with, ranging from creating cures for diseases to genetic manipulation. Apothecaries are also required to inter marines into Dreadnoughts, along with an attending Techmarine, as attempting to get a marine's brain to interface with the chassis has a nasty drawback of killing the recipient if not handled correctly.
That's not to suggest they don't actually know any medicine, by the way. The Black Library Writers felt insecure, so they put a scene in "Purging of Kadillus" where an Apoc does leg surgery in the middle of a firefight. The RPGs also suggest that Apothecaries are versed in the specific use of poisons and toxins, preparing the chapter's hellfire rounds.
Fun fact: back in the days of Rogue Trader, they actually were called Medics for a while, but that quickly changed.
Tabletop
7th edition and before
One ordinary Command Squad marine who trades his bolt pistol to grant their squad Feel No Pain. That was pretty much it.
8th edition
| Pts/PL | M | WS | BS | S | T | W | A | Ld | Sv |
| 55/3 | 6" | 3+ | 3+ | 4 | 4 | 4 | 3 | 8 | 3+ |
Now treated as a proper veteran, Apothecaries trade their classic Feel No Pain for actual healing. Once per Movement phase an Apothecary can heal a model for D3 wounds. Alternatively it can roll a die to revive a fallen model, placing it back on the field with 1 wound, provided it rolls a 4+. If this fails the Apoc spends the rest of the shooting and assault phases doing absolutely nothing, to represent him harvesting gene seed.
Chaos Apothecaries
Chaos being what it is, their apothecaries range from nonexistent to derp to mad scientist. Between the lack of institutionalization and the high priority placed on Rip and Tear, Chaos apothecaries are rare to nonexistent within a single warband. That said, that's not to say they don't exist, but Chaos apothecaries are too rare to be represented on the tabletop, often being relegated to working on new recruits and playing fluffer to the Chaos Lord instead of the battlefield. Some warbands abandon the apothecary idea completely, using their Dark Mechanicum Biologis allies to harvest and process gene-seed from the dead marines (often including the corpses of loyalists and members of rival bands, if the Lord of the band isn't picky), or even inventing new, more… creative ways to breed new marines.
Few Chaos apothecaries really make a name for themselves. There's that Fabulous Bill guy who has fucked things up for the Imperium with his rampant megalomania and successful genetic experiments (which, by the way, has a huge untapped potential in the fluff). There are also mention of a Nurglite, quasizombified apothecary or two floating around somewhere, and the Red Corsairs having right and proper apothecarion, which might have something to do with Huron Blackheart being the only sensible man in charge on the Chaos side. Also, there is Talos of the Night Lords, who is an ex-apothecarius, as mentioned in the book: Death in the void.
8th edition is here and Mortarion is officially making use of "Chaos Apothecaries" in the form of Plague Surgeons. Nurgle being Nurgle, however, makes their use as "doctors" largely irrelevant (as being a rotten corpse masquerading as a human being means you'll regenerate wounds on your own), and their roles is largely focused on cultivation of various pathogens for the Death Guard to use. Its unknown if Chaos Apothecaries will get any further spotlight, seeing as Tzeentch doesn't need them due to sorcery, Slaanesh probably doesn't either as his/her/it/xe's followers will probably just tell them to embrace suffering rather than cure it, and Khorne will probably be offended at someone needing a doctor when they could just be sucking it up and continue to rip and tear as they die.