An allergic reaction to codeine is relatively rare and can range from mild to potentially fatal. Mild symptoms may include sweating, headache, or skin flushing. More serious signs of an allergic reaction to codeine may involve disorientation, visual disturbances, or hallucinations. Severe reactions may include dizziness, heart palpitations, or difficulty breathing. Any questions or concerns about a potential allergic reaction to codeine should be discussed with a doctor or other medical professional.

Most people who are allergic to codeine will experience symptoms the first time the drug is taken. Occasionally, a person may have a reaction after having taken the medication successfully in the past. Doctors should always be notified if a patient has ever had an allergic reaction to codeine so that alternate medications can be used. A prescription medication known as epinephrine may be prescribed after an allergic reaction has occurred. This potentially life-saving medication is delivered in the form of an injection and is carried with the patient at all times in case allergic symptoms return in the future.

In most cases, an allergic reaction to codeine will cause symptoms to develop within a matter of seconds or minutes following introduction of the drug into the body. A person's exact reaction will depend on the severity of the allergy. The most common symptoms include headache, hives, and mild to moderate swelling. Visual disturbances or excessive sweating may also be present.

More serious signs of an allergic reaction to codeine may include heart palpitations, extreme dizziness, or changes in blood pressure. In some cases, the patient may become disoriented or go into shock. Cardiac arrest is not a common symptom of an allergic reaction to codeine, but it is possible for an allergy to codeine to lead to heart problems such as cardiac arrest or sudden death.

Severe and potentially life-threatening signs of an allergic reaction to codeine include facial swelling, difficulty breathing, and a loss of consciousness. This type of severe allergic reaction is known as anaphylaxis and can prove fatal within a matter of minutes, especially if immediate medical assistance is not obtained. Hospitalization is usually needed for this type of reaction, and the patient may need mechanical help breathing or other life-support measures. A person who has had an anaphylactic reaction to codeine should be careful to avoid this drug in the future, and all members of the medical staff should be reminded at each visit that a codeine allergy exists.