Slight burning, tingling, or stinging may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. If your doctor has directed you to use this product, remember that your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this product do not have serious side effects. This product has rarely caused a very serious (possibly fatal) blood disorder (methemoglobinemia). This effect is more likely if you have breathing problems, certain diseases of the blood, or if you smoke (see also Precautions section). The symptoms of this disorder may occur within minutes to a couple of hours after using this product. Stop using this product and get medical help right away if you develop any symptoms of methemoglobinemia, including: pale/bluish/gray skin, unusual tiredness, shortness of breath, fast heartbeat, dizziness, headache, confusion. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.