Lactarius piperatus, commonly known as the peppery milk-cap is a semi-edible basidiomycete fungus of the genus Lactarius. Despite being edible, it is not recommended due to its poor taste, though can be used as seasoning when dried. The fruiting body is a creamy-white mushroom which is funnel-shaped when mature, with exceptionally crowded gills. It bleeds a whitish peppery-tasting milk when cut. Widely distributed across Europe and eastern North America, Lactarius piperatus has been accidentally introduced to Australia. Mycorrhizal, it forms a symbiotic relationship with various species of deciduous tree, including beech, and hazel, and fruiting bodies are found on the forest floor in deciduous woodland.
L. piperatus has a cap that varies from 6–16 cm (2.4–6.4  in) across and is convex with a widely funnel-shaped centre. The cap is creamy-white in colour, glabrous and not glossy; its surface may become cracked in dry locales. The stipe is white in colour, smooth, 3–7 cm (1.4–2.8 in) long by 2–3 cm (0.8-1.2 in) thick and is cylindrical, sometimes tapering towards the base. There is a thick layer of firm white flesh, and the decurrent gills are particularly crowded and narrow, sharing the white colouration of the stem but becoming creamy with age. As with other species of Lactarius, there is abundant milk (latex), which is white, and dries olive-green. It has a white spore print with elongate, elliptic or amyloid spores which are ornamented, as with L. vellereus. The spores measure from 6.5–9.5 by 5–8 μm, and have tiny warts.
L. vellereus is larger with a thick stipe, woolly cap and less crowded gills, but is not as tall. Russula delica is similar in colour and shape, though has adnate blue-green tinged gills and no milk. L. deceptivus is also similar, but is differentiated by its less crowded gills, firmer cap margin and less acrid milk. Variants of L. piperatus include L. piperatus var. glaucescens, sometimes known as L. glaucescens, which is differentiated by its milk, which dries a greenish colour.
L. piperatus is found in Europe, the Black sea region in northeastern Turkey, and eastern and central North America east of Minnesota. It has been accidentally introduced into Australasia, where it is found under introduced and native trees. It is found on the floor in deciduous woodland, particularly beech, and can be found throughout summer and autumn and into early winter. It is relatively common, though not as common as its cousin L. vellereus. L. piperatus is found solitarily or in scattered groups. It is sometimes found growing together with Russula cyanoxantha.
Despite being described by some mycologists as inedible or even poisonous, L. piperatus is generally considered edible. However, it is not recommended, due to its unpleasant taste. It is difficult to digest when eaten raw, but is used as a seasoning when dried, or sometimes is eaten fresh after parboiling, though its taste is still unappetising. Some recommend frying in butter with bacon and onion or pickling, or baking in a pie or pastry as other ways to prepare it. The milk has a very hot and acrid taste, which is removed if boiled. The mushroom used to be highly regarded in Russia, where it would be picked in dry seasons when other edible species were less available. The mushroom is also popular in Finland, where cooks boil it repeatedly, disposing of the water each time, and then store it in salt water and keep it refrigerated, after which it is pickled or served in salads. When eaten fresh and raw, the mushroom has been known to cause an irritant reaction on the lips and tongue, which subsides after an hour. L. piperatus var. glaucescens has been reported to be poisonous, but it has been speculated that the "poisonings" were caused by the extremely strong, peppery taste, rather than by the presence of actual poisons.
L. piperatus forms part of an unusual and highly regarded dish in North America, being one of species several colonised by the lobster mushroom Hypomyces lactifluorum. This results in an orange-pink crusty outer-skin which improves the taste. It is also a common source of food among red squirrels.
Due to the presence of auxins in L. piperatus metabolites, it can be applied as a rooting hormone to aid the growth of seedlings of various species of plants, including hazel, beech and oak. In the 19th century, it was also used as a folk cure for tuberculosis, though it had no effect. In more recent times, it has been found that L. piperatus can be used as an antiviral agent, and the latex has been used against viral warts.