(Novel Research!) Medieval poisons and cures?

You'll probably want some form of muscle relaxant (nervous signal blocker) or sedative (general weakness). Alternatively, you can pick an arbitrary (known/real) chemical and have it cause paralysis to dragons only, similarly to how chocolate is highly toxic to dogs (Theobromine poisoning). Or, you can create a fictional poison solely for this purpose. It should however be noted that the larger a creature is, the more of a chemical is needed to. For example, the amount of sedatives needed to knock out a full grown elephant can potentially kill a human baby.

Regarding water & poisoning from a biological standpoint, assuming the damage is non-permanent, effects will wear off as the poison is flushed out through the body through excretion (kidney, sweat, ...). However, this will cause a significant loss of water to return the body back to homeostasis (aka: body norm), resulting in dehydration. As such, drinking plenty of ordinary water will (a) increase rate of excretion removing the poison out of the body faster (to a certain limit), (b) decrease the risk of kidney/bladder/... damage from high levels of toxic material there, and (c) prevent dehydration in general. This means that it really is more of time being the cure than plain old ordinary water itself. This can best be seen in alcohol, which the human body considers as a toxin.

Asking random strangers on the internet, after doing 15 seconds of gloobling, is not "research," little child.

Go to the library and read some books by experts on these subjects. If you can't find what you need, ask a librarian to help you. They can find all sorts of stuff -- online as well as in print -- that glooble will never show you.

Okay so the fact that you've got dragons in your story makes me assume it's fantasy right? If you're having trouble figuring out what poison to use why not make it up? It's called artistic license....basically it's your story, write what you want to.