If you really want to bring the real world into a discussion that used to be entirely about the rules cosmos, think about this:
"The dose makes the poison." -- Paracelsus
As we've been discussing, that's actually quite an interesting game mechanics point. Almost all good drugs have a harmful or fatal overdose. Some drugs have a pretty narrow margin between helpful and fatal. If you postulate a magical mechanism that differentiates between "medicine" and "poison", you have to assume some degree of circumstantial judgment and/or precognition, since both are likely to be true for a given substance. Essentially the magic would have to work intelligently, which could open up all sorts of interesting potential plot doors.
Did you know oxygen caused cancer? Or sawdust? Did you know that carbon dioxide is quite, quite poisonous to most animal life in sufficient quantity, even though plants thrive on it? Pure water, ingested in great enough doses, will kill you by inducing osmotic imbalance in your intestines.
....

I don't think you have a very clear understanding of neoplastic processes. This is D&D Legacy, not Bio 101, so let's just never mind.
So is a Druid immune to those? What does immunity mean in that case? Can't a Druid breathe, because inhaling oxygen might be bad for his health? Can't a Druid drink too much water?
Again we're back to the definition of "immunity" - is it MAAAAGIC and working by an unknown, godlike mechanism, or has the DM chosen to flesh out their world background with a bit of poison lore creatively mined from our world's science? Either way is fine, really - it's a game, and ultimately that creative control belongs to the individual DM. If you are tweaking the basic rules recipe with a tasty scoop of real-world poison lore, then you may want to deal with the "medicine is poison" question a little more realistically.
In the real world, "immunity" should probably be written as "resistance". It's not magic, it's antibodies. Whatever quantity of antibodies a given organism has in circulation will bind to an equivalent quantity of foreign bodies in the bloodstream and neutralize them. If there's more antibodies than venom, you're good. If there's more venom than antibodies, that's bad, and you will see some envenomation effects occurring at least until those antibodies can be replenished. The damage already done won't be reversed, but the new antibodies going into circulation can continue their neutralization work.
We could decide that a magical ritual or monk-like body control discipline taught to druids can speed up antibody production or boost its circulating levels, or maintain them in the absence of biweekly injections. That's pretty consistent with both science and the fantasy world background we're playing in. Whether you care enough to bother with this level of detail in fleshing out your druid character background is entirely up to you.
If you do go this route, you might also want to think about whether your druids would be immune to poisons that are not affected by antibodies, or against which they would not reasonably be able to raise antibodies. Again, potentially fun plot hooks here. An order of Druids that went to great lengths to instill strong resistances vs natural toxins in their members might be awfully surprised by a rival sect of assassins using a non-local type of venom that they hadn't had a chance to develop immunity against.
Or they might get caught with their britches down by a poison type that ignored antibodies. That lead glaze recently invented by the local monastery is really lovely, and trade in it has benefited their economy greatly. How odd that people are dying mysteriously, including a Druid of the local Order, who of course couldn't possibly have died from poison, given the potent immunities conferred by his mystical practices. Shall we investigate?
Plot hooks. Fun stuff. This is pretty much what I play D&D for. Your mileage may vary, of course.
And don't get me started on drug abuse and the question which drugs are socially unacceptable, thus "bad", thus "poison", and which drugs are socially acceptable. Alcohol is the usual example given (or marijuana in some nations), but let's think out of the box: did you know people can become addicted to sugar? A lot of Inuit have a huge sweets habit, which WILL kill them or at least make them sick if they can't kick it.
And there's another great plot hook. If some well meaning druid created a magical poison immunity talent/item/potion/spell/etc that operated on morals, and had something of an imperfect "drug vs poison" filter on its judgment calls, can you imagine how little fun your characters would have in a tavern? Or a kitchen for that matter. And, no medicine for you! Ever. Maybe no food either. Certainly no herbs or spices or salt. My, that could be fun. For the DM, anyhow. Muahahaha. (insert evil DM laugh here)
See, a poison is quite clearly defined in D&D rules texts, but in the real world, there usually isn't such a thing as a clear definition, even though scientists like to carry themselves as if everything were clear-cut. So please don't bring in your real-life categories. They have no bearing on the question which substances a Druid is immune to in D&D.
Quite a lot is known to science, if not necessarily to you personally. While no one needs a D&D game to be a National Geographic documentary, there are all kinds of interesting places a DM can go looking for nifty bits of lore, plot hooks and background information to flesh out their world and character background. It's strictly optional, and you can always go with the simple standard rules on poison if you have neither the time nor the inclination to do otherwise. Nothing wrong with that; it's a game, and the object is to have fun, not to tie yourself in knots pondering things that don't interest you.
But, for folks who really are interested and see some potential for creative worldbuilding by fleshing out house rules with a bit of realism, this kind of stuff is good geeky fun and good background material.
That is all. Have I mentioned that this stuff is optional? If you think it's nifty and want to use it as world background or a plot hook, great. If it's not relevant to your interests or your campaign, don't use it. But please don't begrudge others the right to use it or brainstorm about it if it does look like good campaign material to them.