Most people who think they're allergic to codeine aren't actually allergic. An allergic response involves your immune system; antibodies called immunoglobulin E are released and they trigger your mast cells and the production of histamine. People with true allergies to codeine, which is an opioid, can suffer symptoms like hives, rash, low blood pressure, difficulty breathing, and swelling beneath the skin (angioedema). However, most people with codeine "allergies" really suffer from a pseudoallergy that doesn't involve the immune system but can involve histamine anyway. Symptoms of codeine pseudoallergies are flushing, itching, sneezing, hives, sweating, low blood pressure and worsening of asthma. The difference between the allergy and a pseudoallergy is that an allergic response can be caused by even a minute amount of codeine, whereas a pseudoallergic response is typically dose-related.
In addition, codeine can carry side effects even for people with no specific sensitivity. These include drowsiness, nausea, constipation, itching and sometimes even difficulty breathing.