Er... I don't think Chaos really need apothecaries... Mainly because well... considering Khorne likes blood and they bleed blood, the more the bleed the more I guess they're blessed by Khorne? I'm not sure :/
http://wh40k.lexicanum.com/wiki/Daemonculaba#.UngDW_ku57M looks like the dark mechanicus provide the apotecary role for the chaos space marine.
Technically, they still use apothecaries, or something of a medic role, but they're generally more towards the undivided chapters, or ones who aren't huge on worship. Night Lords, Red Corsairs, Blood Gorgons, Word Bearers all have had novel mentions of apothecaries retained in the their ranks (of some form). But to make them feel different enough in game (chaos versus loyalists) some lore will have to be neglected.
The fluff mentions them from time again and again here and there, just they have not been getting a lot of attention though. However they do exist i do believe they are working on the sidelines with experimentation and the like, so suffice to say.... According to the book "Storm of Iron" with word bearers they are referred to as "Chimeruks". Now we know how chaos replenishes their forces, the apothecaries are very well alive and kicking forth more soldiers for chaos, like that fabius bile who was once chief apothecary pre-heresy times. So apothecaries are protected and are doing their job´s in the end, like good ol fabius. Here is the wiki regarding Fabius Bile: "He was nearly caught by the Salamanderschapter in the Arden system and has now set up base on one of the EldarCrone Worlds within the Eye of Terror. He has forged alliances with the various Chaos Space Marine legions and promises to give them warriors to replenish their forces in return for protection."
Yeah, there we go. The Blood Gorgon's equivalent are called Chirurgeon, or Chirurgeon witches. Chirurgeon‐Witches - The Chirurgeons are responsible for tending to the Chapter's wounded and recovering the gene-seed from fallen Bond-Brothers, much like the Apothecaries of a Loyalist Space Marine Chapter. They are also responsible for performing the blasphemous ritual of Blood-Bonding. They are known to be powerful psykers who practice the dark arts of necromancy and Chaos Sorcery, maintaining the heretical rites and rituals of Chaos Undivided. This small coven of witches are not Blood-Bonded like their brethren, and from this difference grows a rift between the witches and their brethren among the Chapter's companies. It is a respectful rift but a rift nonetheless. As Araghast said, they exist in the fluff, they're just most often experimenting, busy recreating more astartes, or dabbling in warp magic.
If you remember the Night Lords series, Talos was a Apothecary once. So in some way they do still have them just in different types of role.
