Microwaving is probably the most efficient way to reduce agaritine levels in fresh mushrooms.
There is a toxin in plain white button mushrooms called agaritine, which may be carcinogenic. Plain white button mushrooms grow to be cremini (brown) mushrooms, and cremini mushrooms grow to be portobello mushrooms. They’re all the very same mushroom, similar to how green bell peppers are just unripe red bell peppers. The amount of agaritine in these mushrooms can be reduced through cooking: Frying, microwaving, boiling, and even just freezing and thawing lower the levels. “It is therefore recommended to process/cook Button Mushroom before consumption,” something I noted in a video that’s now more than a decade old.
However, as shown below and at 0:51 in my video Is It Safe to Eat Raw Mushrooms?, if you look at the various cooking methods, the agaritine in these mushrooms isn’t completely destroyed. Take dry baking, for example: Baking for ten minutes at about 400° Fahrenheit (“a process similar to pizza baking”) only cuts the agaritine levels by about a quarter, so 77 percent still remains.
Boiling looks better, appearing to wipe out more than half the toxin after just five minutes, but the agaritine isn’t actually eliminated. Instead, it’s just transferred to the cooking water. So, levels within the mushrooms drop by about half at five minutes and by 90 percent after an hour, but that’s mostly because the agartine is leaching into the broth. So, if you’re making soup, for instance, five minutes of boiling is no more effective than dry baking for ten minutes, and, even after an hour, about half still remains.
Frying for five to ten minutes eliminates a lot of agartine, but microwaving is not only a more healthful way to cook, but it works even better, as you can see here and at 1:39 in my video. Researchers found that just one minute in the microwave “reduced the agaritine content of the mushrooms by 65%,” and only 30 seconds of microwaving eliminated more than 50 percent. So, microwaving is probably the easiest way to reduce agaritine levels in fresh mushrooms. 
My technique is to add dried mushrooms into the pasta water when I’m making spaghetti. Between the reductions of 20 percent or so from the drying and 60 percent or so from boiling for ten minutes and straining, more than 90 percent of agaritine is eliminated.
Should we be concerned about the residual agaritine? According to a review funded by the mushroom industry, not at all. “The available evidence to date suggests that agaritine from consumption of…mushrooms poses no known toxicological risk to healthy humans.” The researchers acknowledge agartine is considered a potential carcinogen in mice, but then that data needs to be extrapolated to human health outcomes.
The Swiss Institute of Technology, for example, estimated that the average mushroom consumption in the country would be expected to cause about two cases of cancer per one hundred thousand people. That is similar to consumption in the United States, as seen below and at 3:00 in my video, so “one could theoretically expect about 20 cancer deaths per 1 x 106 [one million] lives from mushroom consumption.” In comparison, typically, with a new chemical, pesticide, or food additive, we’d like to see the cancer risk lower than one in a million. “By this approach, the average mushroom consumption of Switzerland is 20-fold too high to be acceptable. To remain under the limit”—and keep risk down to one in a million—“‘mushroom lovers’ would have to restrict their consumption of mushrooms to one 50-g serving every 250 days!” That’s about a half-cup serving once in just over eight months. To put that into perspective, even if you were eating a single serving every single day, the resulting additional cancer risk would only be about one in ten thousand. “Put another way, if 10,000 people consumed a mushroom meal daily for 70 years, then in addition to the 3000 cancer cases arising from other factors, one more case could be attributed to consuming mushrooms.” 
But, again, this is all based “on the presumption that results in such mouse models are equally valid in humans.” Indeed, this is all just extrapolating from mice data. What we need is a huge prospective study to examine the association between mushroom consumption and cancer risk in humans, but there weren’t any such studies—until now.
Researchers titled their paper: “Mushroom Consumption and Risk of Total and Site-Specific Cancer in Two Large U.S. [Harvard] Prospective Cohorts” and found “no association between mushroom consumption and total and site-specific cancers in U.S. women and men.”
Eating raw or undercooked shiitake mushrooms can cause something else, though: shiitake mushroom flagellate dermatitis. Flagellate as in flagellation, whipping, flogging. Below and at 4:48 in my video, you can see a rash that makes it look as if you’ve been whipped.
Here and at 4:58 in my video is another photo of the rash. It’s thought to be caused by a compound in shiitake mushrooms called lentinan, but because heat denatures it, it only seems to be a problem with raw or undercooked mushrooms.
Now, it is rare. Only about 1 in 50 people are even susceptible, and it goes away on its own in a week or two. Interestingly, it can strike as many as ten days after eating shiitake mushrooms, which is why people may not make the connection. One unfortunate man suffered on and off for 16 years before a diagnosis. Hopefully, a lot of doctors will watch this video, and if they ever see a rash like this, they’ll tell their patients to cook their shiitakes.
