Ganoderma, or Reishi mushroom, is a Japanese mushroom used as an herbal supplement. Alternative medicine users tout ganoderma's healing qualities and claim it can fight allergies, lung problems, liver disease, cancer, diabetes, heart disease and other ailments. While taking ganoderma supplements may confer a health benefit, the substance is not without its risks.Some people are allergic to ganoderma or experience a hypersensitivity to its components. The most common side effects of ganoderma use in hypersensitive individuals is a red, itchy skin rash. According to Right Health, other users reported dry mouth, nosebleeds and throat dryness following ganoderma use.According to the University of Michigan, ganoderma can interfere with certain prescription medications. If you take medication to control diabetes, consult a doctor before taking a ganoderma supplement. The interaction of ganoderma with diabetes medication can cause a dangerous drop in blood pressure.Ganoderma causes thinning of the blood, which can be healthy for some individuals with high blood pressure. However, long term use may lead to gastric bleeding, a potentially dangerous side effect. Lower doses of ganoderma sometimes cause diarrhea or bloody stool. People taking aspirin or anticoagulants should avoid the use of ganoderma, as it may result in dangerously low blood pressure.