Patients taking furadantin may experience side effects. The common side effects are usually not serious, but if a patient experiences the serious side effects or if the common ones persist or worsen their should immediately contact their physician. The common side effects include nausea, drowsiness, vomiting, dizziness, headache, and loss of appetite. Some patients may notice that their urine turns brown or dark yellow. In most cases, this side effect is harmless, however it may also indicate a serious problem so patients should alert their doctor if they experience this side effect, especially if it accompanied by yellowing of the skin or eyes, persistent vomiting/nausea, fatigue, abdominal or stomach pain, or a pounding/fast heartbeat. The serious side effects include eye pain, bruising or bleeding easily, vision changes, a new infection, severe/persistent headaches, and mental/mood changes. Certain side effects warrant immediate medical attention and these include tingling or numbness of the feet or hands, purplish/bluish skin, unusual muscle weakness, persistent cough, difficulty breathing, shortness of breath, or chest pain.