Shiitake mushrooms are a type of edible fungus that is native to Asia and can be consumed raw, cooked, or in nutritional supplement form as a powdered extract of the mushroom's contents. Shiitake mushrooms have been used as an herbal remedy for a variety of ailments, and is commonly used in Asian cooking. Despite its benefits, some adverse effects of shiitake mushroom intake, either in whole or supplement form, have been identified, according to the book, "Today's Herbal Health: The Essential Reference Guide." If you still decide to use a shiitake mushroom supplement, or add large quantities of shiitake mushrooms to your diet, consult your physician first.
The American Cancer Society notes that shiitake mushrooms are generally considered safe, although they may cause digestive distress, including bloating and diarrhea. Other digestive side effects include upset stomach, nausea, persistent stomach cramps, and vomiting. These effects are typically associated with a large intake of shiitake mushrooms in a single serving and is most likely related to your body's inability to process the contents of the mushroom all at once.
Shiitake mushrooms can cause an allergic reaction in some individuals, which can adversely affect your skin, nose, throat, or lungs, according to the American Cancer Society. Common symptoms of an allergic reaction to mushrooms are the appearance of hives, swelling of your face, neck, and throat, difficulty breathing, and an increased heart rate. If you experience any of these symptoms after eating a shiitake mushroom or ingesting a shiitake mushroom nutritional supplement, seek medical attention immediately.
According to a study published in the "Journal of Allergy and Clinical Immunology," daily ingestion of 4 g of shiitake mushrooms over the course of 10 weeks caused blood eosinophilia. Eosinophilia is a blood condition that causes an abnormal increase in the number of a type of white blood cells, called eosinophils, which may be related to the gastrointestinal symptoms associated with shiitake mushroom intake.
Although rare, shiitake mushroom intake has been associated with dermatitis and photosensitivity, according to an article published in the journal of "Dermatology." Shiitake mushroom consumption can cause skin lesions to appear, a condition known as shiitake dermatitis in Japan. The skin lesions associated with shiitake mushroom intake can be made worse when exposed to sunlight, suggesting that the mushroom can cause an increase in photosensitivity.