Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat. Stop using this medicine and call your doctor at once if you have: severe dizziness, blurred vision, fast heart rate; painful or difficult urination; or shortness of breath. Older adults may be more likely to have side effects from this medication. Methylene blue will most likely cause your urine or stools to appear blue or green in color. This is a normal side effect of the medication and will not cause any harm. Common side effects may include: dizziness, drowsiness; dry mouth; or nausea, vomiting. This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.