Setting: Sometime vaguely from 2006 to today, AU but functionally identical to modern-day US (specifically, California, vaguely near the Bay Area) for these purposes.
My MC is being held captive in a very damp environment. His captors want him to be coherent but somewhat pliable (and impaired for this AU's minor magic), so they keep him dosed with a sedative or tranquilizer. (Opiates such as heroin seem less appropriate, but I'm no expert, and I can go with that instead if it works better. I mean any such drug — sedative, tranquilizer, opiate — when I say "sedative" from this point.) I'm thinking they force him to drink it in some fashion on a regular basis (liquid preparation or similar, not trusting him to swallow pills), but I can have them inject the drug instead if that's more plausible. Because they need him capable of devising a plan for them, this can't be an amnesia-inducing drug like GHB. (It's likely the drug will interfere with his concentration; that's unfortunate but probably unavoidable.) They have resources to get any drug they want, whether legal, legal-but-controlled, or illegal.
They don't know — nor care — that the MC has a higher sensitivity to sedatives than the average person does; they would rather resort to a higher maintenance dose than risk the sedation failing, and they're mostly not nice people anyway. I'm figuring he spends at least as much time sleeping as awake because of the sedation. One of the captors is also worried because the MC's breathing is slightly depressed because of the sedation, and between that and the very damp setting she is worried about his lungs. (Does this increase the risk of pneumonia? If so, viral or bacterial or aspirational? If not, what pulmonary illnesses, if any, would he be at greater risk for under these conditions? He is also more susceptible to water-triggered illnesses than the average person, if that makes a difference.)
After several days, fearing they've been discovered, the captors force a potentially fatal dose on the MC and flee, leaving him behind. They mean the dose to be fatal, in fact, but they're not experts so there's wiggle room. I was figuring they would pin him down and inject the dose IV, but IM would be easier if that's actually more plausible. (I'm avoiding PO if at all possible here; I don't want to deal with vomiting or aspiration.) The MC tries to fight the effects but has no chance of succeeding, and he registers the ever-greater difficulty of moving and breathing until he eventually falls unconscious. He has no way of marking time, but for plotting purposes, about how long should I allow for this sequence?
Within minutes — possibly even seconds — of his falling unconscious, though, the other heroes arrive to save the day. While moving him somewhere safer and more accessible to await an ambulance, they notice that he stops breathing. They can provide an oxygen-enhanced form of rescue breathing, and they do so until the EMTs arrive. I'm assuming his pulse is slow and perhaps thready, but his heart does not stop, so no compressions are required. They also have a way to drain some of the excess fluid from his lungs (safely, easily, non-invasively), assuming any has built up from his several days held captive in the very damp environment.
What sort of treatment timeline am I looking at after that point? What I've found so far is a range of observation from two hours to 24 after treatment, and not much more. I wasn't able to find any treatment that would arrest any further progression of the effects of the sedative; would there be such a treatment? (In fact, page 4 of the "How Not to Commit Suicide" link down there gives stats suggesting that drug interventions are more dangerous than waiting out the overdose drug … but it seems to be a bit outdated, and what other treatments would be involved? Would Narcan be an exception — and if so, as a treatment for opiates, is it likely to apply here? What about diuresis?) If he has reached this stage, would they likely have to intubate him, or would they be able to stick to bagging him without intubation? If he's intubated, about how long would he have to remain so, and would he need more observation time later?
I want a realistic but not too onerous recovery; I don't know whether he would need to be in the hospital for two hours or a week (anywhere from overnight to a day or two would best suit my purposes), and I'd like him sent home with someone to keep an eye on him after that. Brain damage is unacceptable; liver damage would be an unfortunate irony, and I'm not looking to include it, but it's not a deal-breaker if it's likely. If he would have any post-discharge requirements, such as medications to treat or ward off early pneumonia or other pulmonary illnesses, that's fine (but is it plausible?); but as far as I can tell, he wouldn't have any particular post-treatment medications or such for the sedative overdose itself. Is that reasonable? (Diuretics?)
(He also likely needs antibiotics for a mild infection stemming from an open injury — a cut that was washed with soap but otherwise untreated — but that shouldn't affect these other elements. I think.)
So, the upshot: What drug class, or if possible specific drug suggestions, am I looking for here? What would the post-overdose treatment timeline look like? What low-grade or nascent pulmonary illnesses, if any, can I plausibly saddle the character with given slight respiratory depression from medication plus a very damp environment?
EMedicine pages on Toxicity, Sedative-Hypnotics; About.com on sedative overdose, an odd fragment from How Not to Commit Suicide; Google Health: Pneumonia; Pneumonia on Wikipedia
Dribs and drabs picked up from various sources over many years;
Google searches on sedative overdose, sedative overdose pulmonary, tranquilizer overdose, damp pneumonia, water in lungs, site:community.livejournal.com/little_de
The "drugs" tag here (this post has similarities but didn't really get answered, but this one makes me wonder if morphine might actually be an option);
Briefly the "medicine (poisoning)" tag here, but it seems differently focused;
Not the other medicine tags here because OMG so I relied on the comm-restricted Google search given above;
The info in this post about secondary drowning
ETA: I poked at Erowid, but I don't think I know enough specifics to use that resource pointfully here.