- Small differences in genes may alter responses to drugs. One gene that has different forms is the mu opioid receptor gene. People with one form of this gene are more sensitive to alcohol. People with a different form are sometimes more sensitive to pain. Morphine and other prescription pain pills produce pain relief by acting at the mu opioid receptor. Researchers want to see how well a drug called remifentanil can help determine sensitivity to pain. Remifentanil is a strong but short-acting pain medication that is sometimes used for anesthesia during surgery. The study will test how much the drug causes the pupils to constrict.