Benzocaine can cause allergic reactions, including anaphylaxis. People with allergies to benzocaine or similar numbing agents, or to PABA (the base molecule of benzocaine), should avoid benzocaine-containing products.
When benzocaine is used in the throat, the numbing action can suppress the gag reflex. Sleeping or unconscious people are put at an increased risk of aspirating vomit or saliva, which can cause airway obstruction (choking) or pneumonia. These products should be used with caution before bedtime.
One of the rare effects of benzocaine is serious medical condition called methemoglobinemia. This condition occurs when the hemoglobin in the blood is converted to a form that cannot carry oxygen to cells. Methemoglobinemia can be triggered by benzocaine when the amount used exceeds the recommended dosage, and is more likely in people with impaired respiration (i.e. smokers or people with asthma, emphysema, or heart disease). Symptoms include shortness of breath, dizziness, and blue or gray color to the extremities (cyanosis). Pulse oximetry, the most common way to monitor blood oxygen in health care settings, is not able to detect methemoglobinemia, which requires specialized tests to confirm.
The Veterans Administration (VA) recently stopped the use of benzocaine sprays in its hospitals because of the risk of methemoglobinemia. Benzocaine sprays are commonly used to numb the throat before insertion of breathing tubes or other procedures. The Food and Drug Administration (FDA) responded with an opinion that the benefits of benzocaine sprays generally outweigh the risks, but care must be taken to use these products correctly.
