Although this species is eaten in some places, in other parts of its range it can have a decidedly acid-sour taste. There are reports that it can produce a gradually acquired hypersensitivity that causes kidney failure.
Not recommended although tasty because it is now known to cause a gradually acquired hypersensitivity that can result in massive hemolysis and be life-threatening.
Good edible, but toxic in the raw state. It is recommended to blanch this mushroom in a very large volume of water for a long time and to throw away the strongly coloured cooking water.
Most of the 15 species of the fungus genus Paxillus are widely regarded as poisonous or even deadly. One of them, Paxillus involutus, is a model species for ecological or physiological studies of ectomycorrhiza, the symbiotic relationship between fungal symbionts and the roots of various plant species. However, assessments whether Paxillus involutus is harmful or not vary depending on different observers' experiences in different countries and such differences in assessments can be the result of undiscovered diversity. After all, there is not even a proper type specimen for Paxillus involutus.
The observation of a large and unusual variation among Paxillus specimens in France led to the suspicion of the presence of a not yet described species and a new publication not only confirms this by using molecular data but also properly describes this new species (Paxillus cuprinus):
Phylogenetic analyses of three nuclear DNA regions (rDNA internal transcribed spacer (ITS), tef1-α, and gpd) confirmed the four European species. Morphology, culture, and ecology features allowed us to delineate species boundaries and to describe the fourth species we named Paxillus cuprinus since it turns coppery with age. As there is no existing original herbarium specimen for Paxillus involutus, one of our collections was chosen as the epitype. The low genetic diversity found in Paxillus cuprinus correlates with stable morphological traits (basidiome colour, ovoid–amygdaliform spores with an apical constriction) and with ecological preferences (association with Betulaceae in open and temperate areas).
For these species the diversity at the genetic level is supported by morphological and ecological variability, often a major obstacle in species delineation. This study establishes species boundaries for the European members of the genus and describes habitat preferences. This knowledge will also help to decrease the amount of accidental mis-identifications that could happen during the next mushroom foray.
Ah yes, in case you didn't know the joke here is the answer to the question in the title: There wasn't mushroom.