Drugstores have exploded into a $252 billion business in the United States, and with over 11 million Americans coming into the healthcare system via Obamacare so far this year, it's only going to get bigger. And yet, the segment's getting smaller all the time—not in dollar value, but brands. If you're on your way to the local drugstore, chances are it's one of three places—Walgreens, CVS or Rite Aid. Now consider this: If regulators green light Walgreens' plan to gobble up Rite Aid, those players will dwindle to two. Few other industries have so few chains competing for so much revenue. So, who's likely to come out on top? And what distinguishes the top two brands, anyway? Below, a snapshot of big pharma's biggest retailers. Might as well get acquainted now, since they'll probably be filling your next prescription very soon.