What about PCs that refuse the drink?
This seems like a weird question to me. This would only happen if you make an issue of it. Part of being a DM is social engineering. Your NPCs cannot make diplomacy rolls to "force" players to like them -- you have to play your NPC as genuinely enjoyable to be around, if you wish the players to perceive the NPC that way. If you wish to pull a bluff, you have to be reasonably good at real-world bluffing. And so on.
Since it's the job, I've gotten fairly decent at it. So for me, I'd gloss over the details to play it off like a standard taken-for-granted event. That way it's a fait accompli. Like this:
DM: The sun is setting. What do you wish to do about food & sleep?
Players: Find an inn.
DM: Okay, sure. The innkeeper offers you rooms, two choices: each pay individually for meals and rooms at the standard rates in the Player's Handbook, or if you're going to be around for a while, he'll give you a group deal but you have to pay in advance. It's 25 gold pieces for all of you for 3 days, meals included.
Players: Uh, that's a savings, so we'll take the 3 days for 25 gold.
DM: Okay. You spend the night. The dinner meal is tasty, and unless you have any last-minute stuff to do, we can fast-forward to the next morning.
Players: We're good.
DM: Great. When you awaken, you are surprised to find yourselves in a jail cell,
unless your character can make a DC 15 Fortitude save
and a DC 16 Will save.
Players: Whaaaat? Hey! One of us made the saves!
DM: Okay, lucky player, you awaken in the morning to find your allies gone. Everyone else, you find yourselves in jail.
It's like a bait & switch. I call attention to the deal they're getting on the inn rates so that they are not paying attention to the fact that they took for granted that they ate the innkeeper's food and slept at his inn. Once I have their buy-in that they accepted the scenario without suspicion, I throw 'em into the action.
After all, eating at the inn and spending the night is something that plays out millions of times in millions of D&D games all over the world. It's absolutely unremarkable. So the only reason that a player would decide not to eat the poisoned food is if you called attention to it. Why would a DM sabotage his own event like that?
(By the way, that's a DC 15 Fort save for Oil of taggit planted into their meals, and a DC 16 Will save for a Deep Slumber spell cast by someone with +3 to their spellcasting stat. Redundancy for the win. And no, I wouldn't tell them "Oil of taggit was planted in your food." I would just make sure that *I* knew it and was aware that the solution was reasonable, so that if they found out, they would see it as plausible themselves.)
What happens when the PCs want to acquire such a poison for their own use? If a lowly guard could afford to use it surely the PCs can get it?
Yes. Good point. Anything the DM uses is fair game for others to use, and if the DM denies it, he or she can expect trouble at the table. So for me, I try to find other ways to handle these things. For example, it seems
eminently fair to use quests -- after all, the PCs get them, so it's reasonable for NPCs to get 'em too. The PCs killed guards, so a quest is awarded to a local NPC hero: 500 gold pieces for each captured member of the murderous gang. A solo wizard at level 9 might hear that and think, "Hmm, if that's 2000 or 2500 for all of them, it might be worth casting a few spells...."
Later, the PCs are asleep at an inn. The NPC wizard uses Dimension Door to accurately and silently beam himself into the room of the PC rumored to be the most weak-willed. He casts something debilitating on the PC, maybe even 2 things if necessary -- deep slumber, hold person, charm monster, and so on. Stuff that's a little bit overkill for lower-level PCs, but still reasonable for a 2500 gp reward. And since he's beaming directly into each PC's room while they sleep, he should probably be able to get at
least the first spell off without resistance.
Once he's done some good work to incapacitate a PC, he ties the PC up, gags the PC, disarms/disrobes the PC, etc. Rinse & repeat with each PC, working toward the most resistant PCs near the end (the ones with good will or fortitude saves). Then he turns them in without delay. He doesn't want his spells to wear off if he can help it. He may have even alerted the guards to his plan, so that the guards can just run into the inn and pick up the PCs like packaged goods.
Depending upon how mean you wish to be, and how simulationist you wish to be, you can even have the wizard keep their stuff. After all, that's what the PCs do when they defeat something. However, while that would make it much easier to justify in terms of why a high-level NPC would take the job, it also will infuriate the players at the table. So be careful with that last bit.
Nonetheless, this will come across as much more palatable to the players, especially if you give them back all/most of their loot. After all, getting a quest seems fair & normal for players, so players are able to swallow the bitter pill when you say, "Well, those in power gave a quest to capture villains. Sadly, you took on the villain role when you killed some guards. You're on the "bad guy" side right now. You'll have to work your way back."
I did this once. I had a fun time, and the players had an "okay" time. They weren't thrilled about getting caught, but also, they enjoyed getting input. I did it like this: I didn't want any player to know what was happening, so I set up a gauntlet of game tables for them to run through. First, everyone sat in my dining room. Nearby is a sliding door to the yard. I invited one player outside, explained what was happening, rolled dice in front of them, and had them respond/react, if the dice allowed it. Then, if the PC was captured, I asked the player to head over to
another door in my home that led to a 2nd table. I had a battlemat, chips, and sodas out on that table. They sat & waited while I ran the
next player through it.
The first couple of players were caught so easily that nothing came of it. However, the 3rd player awakened/resisted and while he was quickly subdued, the player made it clear that he was screaming bloody murder for his allies. I opened the sliding door and said to those who had yet to come outside, "This player would like to tell you something." He then leaned in and said, "We're getting attacked by guards, get your characters ready to fight!" I then sent him to the other table with those captured.
The remaining 2 players were
much more difficult to capture. One ran into the other's room and they ganged up on the guards. They killed a few but were brought down. Once they were down, we all walked over to the 2nd room, joined our friends, and played out the rest of the story from there. Everyone was sorta mellow/bummed for getting caught, but also, since they each had a different experience, there was a lot of talking and sharing of "war stories" as they each recounted what had happened. It was pretty fun after that, and certainly memorable.
