True allergy to narcotic pain relievers is very unusual, but people do have reactions to these drugs. Skin rashes, sneezing and itching can been seen with all of the narcotics. Worsening of asthma symptoms also can occur.
There are very rare cases of allergy to narcotics. Some portion of the chemical structure of the drug itself likely causes the reaction. Not all of the narcotic pain relievers have the same chemical structure. There are basically two different families of narcotic drugs; those related to morphine and those related to meperidine (also known as Demerol). Codeine is a close cousin of morphine. Fentanyl is more closely related to meperidine. That means that an allergic reaction to codeine is not likely to be repeated if fentanyl is used. But true allergic reactions to either are very, very rare.