I met this fella that is something of a mushroom expert. Experience growing all sorts of fungi in a lab and all sorts of indoor/outdoor media. I would guess that he has cultivated dozens or hundreds of different kinds of fungus. He'll eat all sorts of fungus and call it delicious.
But not button mushrooms.
He said that they are toxic. He described the problem, but it went over my head.
He went on to say that it is pretty well known in the mushroom cultivation world. But people will not speak out against them because the industry is operated by .... um .... very nice italian people who tend to wear dark suits and have names like "Vito" and "Guido".
And I've decided I have nothing further to say on this topic.
Well, Wikipedia does have some information about this, leading to a number of primary references like this. The story is that white button mushrooms ("champignons") and their relatives have a relatively high concentration of "agaratine", which is a known carcinogen. Cooking destroys at least some of this compound. Other mushrooms -- for example, Japanese Shitake, Enoki, and Matsutake, have two orders of magnitude less of this stuff.
Huh. So I never heard about this before. Scary. I didn't find any human epidemiology on this.
All mushrooms are toxic, in fact everything is toxic.
Whether you're affected by those toxins (and to what degree) though depends on the dose you receive (either an instantaneous dose or with certain toxins a total dose over time).
Let's take alcohol as an ever interesting example A single glass of beer doesn't affect a lot of people very much. But drink similar volume of 80 proof rum and you're getting serious alcohol poisoning (which is euphemistically called "getting drunk", though the volume of alcohol in half a liter of 80 proof rum would likely have more serious results than a bad headache when applied over a short period of time). Some people will of course be more seriously affected than others.