Devi is a 26 year-old female who is brought to the emergency room because she is feeling difficulty breathing and chest pain. She reports she took over 260 pills of loperamide she bought on Amazon to achieve a sense of euphoria. Her routine urine drug screen is negative for oxycodone, hydrocodone, benzodiazepines and amphetamines. Loperamide can be used because it is a/an