“Treatment” could be as simple as “give the victim plenty of water, and lay them down in a soft bed”. That’ll improve outcomes for a lot of afflictions, and in fact, for some things, that’s about the best you can do. But I don’t think anyone would call water and bedrest an antidote.
I thought an “antidote” for poisoning was to put them under, then pump their stomach with charcoal which absorbs the poisons.
That only works if the poison has been swallowed. You can also breathe poisons, have them injected (e.g. snakebite), or absorb them through the skin.
No problem – you just start over with the next slave, giving them the poison then starting down the list of remaining possible antidotes.
In advance, so you know the typical symptoms of common poisons and the antidpte (if any), for when your enemy tries them on you.
Well that’s an easy fix. Fill the victims lungs and arteries with charcoal to absorb the poison. 
If you’re curious, these were the poisons the ancient Roman doctors identified (This comes from Cillers and Retief’s “Poisons, Poisoning, and the Drug Trade in Ancient Rome”, from the journal Akroterion):
Vegetable:
Henbane, Thorn Apple, Deadly Nightshade, Mandrake, Aconite, Hemlock, Hellebore, White Hellebore, Black Hellebore, Autumn Crocus, Yew, Opium, various mushrooms
Animal:
Spanish Fly, Snakes, Poisonous spiders and scorpions, various marine animals we can’t identify today, Salamanders, Basilisks, the blood of bulls, snakes and gelded goats, the meat of the weasel and shrew, toads
Mineral:
Lead, Arsenic, Gupsos, Mercury, Malachite, various streams in Greece
I don’t think we have antidotes to most of that stuff now. The only ones I know we can definitely make an antidote to with modern science (regardless of whether one is actually commercially available) are the “animal” venoms. And the method we use to make the antidote is to inject the poison into an animal (such as a horse), the centrifuge and use other isolation techniques on the animal’s blood to isolate the antibody to the venom.
There’s a more sophisticated version of the technique called monoclonal antibodies where e-coli or other bacteria are reprogrammed to produce the specific antibody you want and nothing else. This is much better but is expensive.
Let’s see : there are chelating agents that work on heavy metals. But, the one I linked was synthesized in 1957. And I guess we can probably make antidotes to the plant venoms as well with antibodies, maybe.
Anyways, all of this stuff depends on techniques invented in the 20th century.
Regarding the second paragraph: THIS is why I am opposed to giving Narcan out like candy. It should only be used by people who are trained in how to deal with this kind of thing.
If a person’s having an allergic reaction and gets some Benadryl and/or a steroid, it can act quickly or slowly, depending on whether it’s administered PO or IV.
As for last Sunday’s “60 Minutes” piece on ZMapp (RIP Bob Simon 
), not mentioned in the story is that ALL the people who got it had very violent reactions to it (other accounts have said that Dr. Brantly, who was interviewed, was lapsing in and out of consciousness by that point but he did say that he vaguely remembered getting it) and it almost killed Nancy Writebol outright, from anaphylaxis. William Pooley, a British nurse who has since returned to Africa, said the treatment was, for him, almost worse than the disease, but his case of Ebola would probably have been misdiagnosed had he not been in a known Ebola zone because his illness was relatively mild - something that in this context would probably have a very interesting definition.
There are “antidotes” that are given concurrently with medications. The first two that come to mind are mannitol with cisplatin, to counteract kidney damage, and Mesna with ifosfamide, to prevent bladder ulceration. (And yes, I’ve heard Mesna referred to as a “smart drug”. :p) Cisplatin and ifosfamide are highly toxic chemotherapy agents.
And as for one of the best-known antidotes, syrup of ipecac, its use is not recommended any more and I’m not sure you can even get it. If you’re supposed to throw up something poisonous, you probably will anyway, and even bulimics generally wouldn’t use it because the resulting vomiting is so incredibly violent and goes on and on.
With one exception, every time I’ve dispensed Chemet, used to treat lead poisoning (and presumably other HMPs too), has been, with one exception, for a dog. It has to be administered via a tube, because it’s so incredibly foul-tasting and smelling. You can even smell it through a sealed glass jar.
I never had to prepare antivenin, but one of my colleagues had, and she said it took FOR-EV-ER to dissolve, and it was so frustrating because there would be this desperately ill person who needed it, and it just.won’t.dissolve…uuuuRRRGH. Otherwise, it’s quite inert and isn’t harmful.
They give them charcoal in sorbitol. It comes in a container that looks like a caulk tube, and is given PO (by mouth) if the patient is conscious and cooperative, and via tube if they aren’t. Sorbitol is a sugar alcohol that is also a powerful laxative; the charcoal adsorbs the poison (in other words, the poison sticks to the charcoal particles) and the sorbitol shoves it out the other end.
If you’ve ever had diarrhea after eating diabetic candy, you know how powerful sorbitol is.
Romans had several treatments for poisoning. Bleeding and magical amulets were both considered to be effective. Milk and wine were popular.
The famous Roman story about poisoning was that of Mithradates VI of Pontus. Mithradates had, according to legend, made himself immune to pretty much every poison by constantly taking small doses of poisons and thereby making his body accustomed to the effects, and there was a mixture called mithridatium that the Romans used that was supposed to be a universal antidote/protective against poison. (It won’t…the ingredients used tended to be a mixture of herbs and plant resins, none of which are any particular use against poison)
Narcan may initiate withdrawals, but that’s neither health nor life-threatening. While opioid overdose is definitely both of those. I have no problem with lay persons using Narcon on unresponsive people. It won’t do harm (other than temporary unpleasantness like throwing an addict into some withdrawal, which can be very educational) but it can save lives.
And yes, I’ve experienced opioid withdrawal myself. It’s not fun, but it’s ten thousand times better than death.
You will be in a state accurately described as “poisoned, but not (yet) dead” and depending on the exact antidote you may be able to tack on “, with complications” to the end as well. You are unlikely to feel Hollywood Happy for a while.
I know that. I’m talking about the danger to those around the addict, who have to deal with the irate person having their high interrupted.
Alcohol withdrawal, OTOH, can be fatal. I realize you know that, but most laypeople don’t.
AIUI most early (and many modern) “antidotes” were just what they’d use to treat the symptoms of poisoning; i.e. if a poison killed by making your throat swell up, they’d try stuff that works for allergic reactions (whatever they called it before allergies were recognized), “bad air”, and the like.