Eating raw or undercooked Shiitake mushrooms can cause a distinctive whip-like rash in some people called Shiitake mushroom dermatitis. This dramatic rash appears 2-5 days after consumption but is not dangerous and goes away on its own in 10-14 days. The condition is caused by a compound called Lentinan that breaks down when mushrooms are cooked at higher temperatures, so properly cooking Shiitake mushrooms prevents the reaction.