To me, it means potentially deadly. If it is only harmful and not deadly, then I would call it toxic. However, a lot of things can be deadly if taken into extreme excess but are not generally considered poisonous. One or two glasses of wine won't kill you (although they will kill fruit flies), but excessive alcohol can. I've heard of people dying from drinking too much water, or being forced to drink too much water.

In general, I consider something poisonous if it is deadly in small doses, such as arsenic or hemlock.

As a diabetic ... sugar and carbs are poison ...

Celiac/Coeliac wheat is slow poison to gut . . . so sometimes it isn't clear to everyone, what is one person's feast is poison to another.

in my world there are two meanings:

A.) Literally poisonous. Will sicken and/or kill you. Deadly nightshade, arsenic, cyanide. Nightshade is a plant in the same family as tomatoes--don't eat nightshade leaves! Arsenic is a element and found in a number of minerals, sometimes in the soil where a lot of USA rice is grown, and in the rice grown there. You can build up a tolerance if you ingest very small amounts, but people who eat a lot of rice are warned to eat imported rice rather than too much arsenic. Cyanide occurs in peach pits, apricot pits and even apple seeds.

Also as in food poisoning, bacteria and others. E. coli, salmonella, listeria, botulism, anthrax, mad cow, etc. Not the food itself, but something in the food.

Also, severe allergens. Foods that other people get good nutrition and pleasure from that might sicken and/or kill the individual who has an allergy to peanuts, fish, eggs, gluten, etc.

B.) Metaphorical hyperbole. Is so disgusting it makes you feel like it will kill you. E.g., "My mother-in-law tried to poison me with her green bean casserole."

I agree, poisonous means a substance that can kill you, or make you extremely ill although you might survive it.

Toxic substances are something else and food allergies and sensitivities are something else again. My oldest daughter is both celiac and lactose intolerant. So, gluten can (eventually) kill her, making it a poison for her but not for others. Dairy is only going to upset her digestive system, so it's toxic, but not poisonous.

Annie

^^ My definitions are everything Annie said : )

Poisonous is potentially deadly.

I used to love pistachios. They're not poisonous to me since they only make me WISH to be dead, rather than being potentially deadly. They won't kill me, but if I'm a guest in your home, I'll be occupying your throne for a lengthy period of time. None of these will be happy about that (especially if your home houses only one throne.)

(Pine nuts are in the same category. I just did an experiment to discover yes, I can still eat almonds.)

How do I define poisonous?

My good friend's mother in law!

In terms of food, something that can/may/will make you ill or can/may/will cause you to die.