What's good in small concentrations, but lethal in higher? What's a glaring red flag you're encroaching on a lethal concentration?

Just about anything including water or salt

Not just about. Literally everything is lethal at a high enough concentration.
- LD50's are fun!
  https://en.m.wikipedia.org/wiki/Median_lethal_dose
- The first part of the question asks what is safe in small amounts
- I'd argue gravitational force isn't lethal. As long as you don't arrive at whatever is pulling you & the gradient of gravity doesn't change across your body length. You could be perfectly fine (for a while) orbiting a black hole at enormous speeds (assuming you don't collide with matter in the accretion disc.
- I thought about this a bit and concluded that it only applies to physical materials and forces.
  For example: There certainly are lethal ideas, but most of them are not, and much like bosons they can overlap, so filling a person with multiple copies of the same (benign) thought has a diminishing effect.
  But yeah, anything physical has a lethal concentration.

Panadol / Paracetamol / Tylenol / Acetaminophen / C8H9NO2 is exceptionally easy to overdose on. I've done it accidentally a couple of times. It causes liver damage at even lower overdoses, you really don't want that.

The maximum dosage is 1g every 4 to 6 hours, maximum total 4g a day. I am no doctor but I strongly recommend 6+ hours between doses (I set a timer) and I try very hard to not get to 3g or above per day. It's even worse that plenty of medications just throw it in to the mix casually.

Unfortunately as the only first line of defence I have against pain, I cannot avoid it altogether. Redflags for me were light abdominal pain and yellowing of skin under eyes. Plus fatigue, but that's normal in my world.

Huh. That might explain the last two weeks. Dental pains. Lot's of tylenol. And why I feel much better now.
- For dental pain I recommend ibuprofen (advil). Seems to work significantly better than acetaminophen (Tylenol) and seems to be much safer.
- Tylenol can be taken with ibuprofen to increase pain or fever relief. Just follow directions for both.
- Definitely ease up on it if you can, especially if you've been taking it for more than a week regularly. If you can't reduce usage/dose, then be much more generous with the minimum gap between doses, especially if your liver is already under a bit of pressure from other meds or alcohol.
  Pain sucks, and pain management causes pain. Sorry about the dental stuff, my issue isn't teeth but I know that is no fun too.
I accidentally overdosed on acetaminophen after a surgery once. Doctor forgot to tell me (or I was still high when he told me) he gave me acetaminophen during/after the surgery. I thought I still could take up to 4g that day. A few hours after the surgery, the pain started to kick in so I took some acetaminophen. Ended up vomiting uncontrollably.
Can you not use ibuprofen?
- Sadly no, the entire NSAID class are off the table. I'm one of the "lucky" few who gets one of the rare but serious side effects.

Weed. A gram will get you high. A woolpack full of it can crush you like a grape if falling from the hay loft.

    
woolpack
From Middle English wolpak, wullepak, equivalent to wool +‎ pack. 
A bag of wool, traditionally weighing 240 pounds.

TIL, Thx

Ol Willie was right
- Woolie Nelson

According to people I know, I’m only ok in small doses.

Then they leave.

So you've never had so far the opportunity to test whether or not you are lethal at higher doses ?
- I think we have a volunteer

Took a Hazmat class today and the big thing they drilled into our heads was "Everything is toxic at scale." So make anything you want and there is an IDLH concentration.

Age

I think this is one of those "any amount is bad" things.
- Optimal age to be is blastocyst. It's just downhill from there.

dying is a pretty big red flag, you ask me.

Okay. Do you think dying is a pretty big red flag?
- I guess I did ask for that.
  I also guess you like to tell dad jokes.

Botulism can kill you, botox can kill your wrinkles.

And your migraines!

It’s been said a lot that light alcohol consumption is actually good for your health, but this is not actually true. It improves some health outcomes but the benefits are outweighed by the risk. There is no safe amount of alcohol consumption.

Hilarious how the alcohol lobby scratched around for years looking for a good news story about alcohol and came back with some weak sauce link between red wine and heart disease. Meanwhile, 50% of reported sexual assaults are linked with alcohol. Probably more like 80%. How is alcohol legal and LSD is a schedule 1 substance? There is no lethal dose and has been discovered as a treatment for resistant depression and PTSD, OCD, etc. Our drug laws are completely wack.
- The real elephant in the room is alcohol is much more destructive and just as addictive than measurable controlled doses of opoids, and also equally if not more destructive than stimulants like amphetamines and cocaine. It's basically the worst drug in every metric. If every alcoholic got their drinks replaced with MS Contin and or weed, the strain on the health system would be much less. Not to mention all the other negative externalities related to alcohol intoxication.
  I say all this as someone who drinks. I am going to ensure my child is aware of all the negatives at an early age and be open to discussion about substances. People either like or dislike alcohol, and the people who like it have no self control. It's a stupid drug, I wish I was never introduced the way I was.
  Don't even get me started on the alcohol companies producing alcoholic mountain Dew, freeze pops and Sunny D... and they have the gall to ban flavored vapes haha.
- Yep and plenty of folks wanted to believe it.
Did anyone not know this at this point? I'm still gonna drink.

Everything is good in moderation. Even drinking too much water can lead to death.

ionizing radiation, according to some hypothesis, vitamin E, selenium, zinc,

there's no single "red flag" everything is different

Everything. Literally everything can be fatal in large enough quantities. Quote from one of my chemistry professors: "there are no lethal substances, only lethal doses"

Even water can be fatal if you drink too much in one sitting.
- Remember the Wii craze in 2007? There was a radio competition called Hold Your Wee for a Wii in which the contestants had to drink throughout the day without going for a piss. One died.
"The dose makes the poison" -some old dude that like found the pharmaceutical industry

There are lots of things that our body needs in really small doses. But anything above can be lethal.

Some things needs to be in really specific compounds. Like chrome we need really small dose of Cr3+ but Cr6+ is carcinogenic.

Water. You need it to live, but you can also drown in it or even drink too much and dilute your blood and die.

https://en.wikipedia.org/wiki/Death_of_Leah_Betts
Yeah this just goes to show how the answer to OP’s question really is “everything.”

Acetaminophen/Paracetamol. The safe therapeutic dose is very close to the toxic dose. While most people don't intentionally overdose, at least not for treating illness symptoms, the problem arises when they take multiple medications that all contain acetaminophen, following the label for all of them can easily net you a toxic dose.

Chubbyemu video: https://www.youtube.com/watch?v=VSqrCgFMsCI

Some good fan service in that video

My medication. Lithium in small exactly precise doses according to each individual is helpful. But we have to take blood tests to check the toxicity.

We have to stay hydrated and be really careful if we are sick or vomiting

A warning signs is shaking (I tests my hands regularly), nausea and outcome is dying. Ha.

Bipolar?
- Yes

The human body is pretty weak so anything in high concentrations can probably kill us. It's no steel

Cocaine. A red flag being your heart beating itself to death inside your chest.

Huh. I kinda thought that was supposed to happen.
- Definitely better inside your chest than outside it. So I hear.

Salt.

Stupidity

Some might say it's not lethal enough given how many absolute morons are still around!

Heartbeats. 60 - 70 is good (you can get away with lower if you're fit, but anything below 40 is bad), and anything above 180 - 200+ tends to be quite bad for your health.

Is there a lethal heartbeat? Google didn't immediately tell me.
- Yes. Anything above 220 tend to be quite bad for your health, since there's not enough time for your heart's chambers to fill at that speed.
  As usual with biology, there's exceptions to the rule, but for your average human, anything past that means that there's usually not enough blood coming out to keep you alive at those speeds, and your heart may as well not be beating at all, for all the good it does.
  It's similar to why Ventricular Fibrillation is bad for you, since it's a similar problem (but only limited to the two main pumping chambers/ventricles of the heart, rather than including the atria).

Vitamin A. Probably others as well.

Also, water.

Blood sugar. Vital that you have enough to continue functioning but if it stays too high for too long, it will disable and eventually kill you.

Caffeine.

What's the LD50 of the internet? 🤔

Reddit
I dunno, but I suspect we've all surpassed it 😅

Elephant steak vs elephant

Nicotine, small doses can be okay, but large doses and it's really lethal

Many plant-based drugs, including nicotine, originated as natural insecticides: the plant uses them to make itself poisonous to insects that would eat it.
But impossible to overdose by voluntary inhalation. Just don't eat, drink or inject lethal doses and you'll be fine.
I'm not sure bout that. I know they say it, but on many occasions I've taken 80+mg orally in a day. Lethal dose is supposed to be 60.
- Ld50 is usually in units per kg, so unless you are a pixie, 90 is well under that number.
  Remember that the ld50 is lethal for 50% of people (healthy adults? Something like that.)

Booze... Nurofen

Apparently there is no longer a lower limit where alcohol is considered beneficial, at all.
https://www.who.int/europe/news/item/04-01-2023-no-level-of-alcohol-consumption-is-safe-for-our-health
- I was thinking more the "merry" side of good rather than healthy good

LSD

There is no lethal dose of LSD
- I'm sure drowning or inhaling it would be fatal. Or something similar to drinking too much water.
- I had a friend that od'd on LSD. His brain swelled up and actually cracked his skull. He was only 20. His sister rushed him to the hospital, but he was brain dead by then.