'Bad Medicine' is a Dark Brotherhood quest in Oblivion Remastered. It is, in our minds, one of the more challenging ones to do "by the book", which is to say, stealthily in order to earn your bonus reward. Doing so is highly advised, as 'Bad Medicine' will net proper assassin types who follow orders to the letter a unique outfit with some genuine utility.
Read on to get the scoop on the best way to venture through Fort Sutch without being detected. While you needn't do more than slay the opposition here, you really ought to please Ocheeva.
How To Start 'Bad Medicine'
Once you have obtained 'Enter the Eliminator', the next automatic mini-quest in the Dark Brotherhood faction, do the usual song-and-dance, which is to say, speak with Ocheeva.
Our target this time is Roderick, and he's located at Fort Sutch. While the game will give you a map tag for the location, we've found that the easiest way to get to Fort Sutch is to fast-travel (or travel traditionally, if you're like us, and would prefer the more immersive approach) to Anvil.
You'll notice that the objective marker is due north from Anvil, or at least, close enough. This direct path is the best path. You'll find the doorway into Fort Sutch just southeast from the map marker itself.
The Poison In The Cupboard
Sneaking through Fort Sutch without being detected is one of the tougher gigs you'll do for the Dark Brotherhood. At least, it will be if you don't have Invisibility or 
Chameleon. You can get the latter free of charge for 60 seconds per day if you chose the Shadow birthsign.
But, since we reckon most players will have chosen something else, you can always craft it through Potions and Spellcraft. Better still, Mraaj'Da will sell both Invisibility and Chameleon spells back at home base. You will, of course, need a high enough Illusion stat to cast them.
Now, here's the thing: you can, in fact, kill the enemies here. You simply need to do so whilst remaining undetected. Since this is possible through a myriad of means, including well-timed arrows and sneak-up blade assassinations,
On the other hand, we've actually found it easier to get through Fort Sutch undetected via aforementioned Invisibility or Chameleon means. Regardless of your choice here, the pathways in Fort Sutch are generally dim enough that you have an advantage in remaining undetected.
This means you needn't have a sky-high Stealth stat to make the magic happen, so to speak, but we'll always recommend cranking up Stealth as much as possible; remember that you can always level it up (albeit glacially) by sneaking past and quietly hiding from regular NPCs throughout Oblivion.
Head toward the left side of Fort Sutch, in the far back, to locate Roderick's Quarters. Here, you'll find the cabinet with Roderick's medicine, where you must place the poison and let time do its deed.
You can kill Roderick outright if you don't care about the bonus reward (and the role-playing perks of Ocheeva's satisfaction, for that matter). That's the thing about most Dark Brotherhood quests: if you really do not care about being a sneaky assassin, you can still progress the questline!
Return to Ocheeva for your leveled gold reward. For your bonus - and this is why we do strongly suggest following the stealth route! - you'll get the unique The Deceiver's Finery outfit, which comes with 
Fortify Personality and Fortify Speechcraft.