Morphine, a sedative, is often used to treat pain. While true morphine allergies are relatively rare, adverse responses to morphine are more common.
A “true” allergic reaction has to do with the immune system. If your immune system does not respond well to morphine, alternate drugs, including a different class of opiates may be prescribed to handle the pain you’re experiencing.
Three different classes of opioids, which are commonly used in the treatment of pain are: phenanthrenes, phenylpiperidine, and phenylheptane. Morphine, codeine, as well as oxycodone, and hydrocodone all belong to the phenanthrenes class.
A patient may be prescribed a drug from another class of opiods, such as fentanyl, which is from the phenylpiperidine class, as a morphine alternative. Fortunately, if a patient suffers an adverse reaction to morphine or codeine, he or she will rarely exhibit the same symptoms from a drug in another opioid class.
https://www.empowher.com/morphine-allergy/content/things-know-about-morphine-allergy