Codeine allergy
Allergy or sensitivity to codeine.
About codeine allergy
Codeine is an opioid painkiller, often combined with paracetamol, and is also used to suppress coughs. Many reactions attributed to it are not true immune allergy. Opioids can directly prompt certain cells to release histamine, causing itching, flushing or a rash without the IgE mechanism of genuine allergy; nausea, constipation and drowsiness are pharmacological side effects rather than allergy at all. Codeine is also converted by the body into morphine, and people vary genetically in how quickly they do so. Genuine IgE-mediated codeine allergy exists but is uncommon.
When reactions occur they are often mild: itching, a blotchy rash, hives or localised swelling from histamine release, and the onset described spans minutes to hours with some rashes appearing after days. True allergic reactions can be more serious, and anaphylaxis is possible though rare. Because the effect is partly a class characteristic, someone who reacts to codeine may also react to related opioids.
Management depends on distinguishing a predictable side effect from genuine allergy, which shapes whether related painkillers can be used. Tell every doctor, dentist and pharmacist, since codeine appears in several combination products. Specialist assessment can clarify it and identify safe alternatives. A severe reaction with breathing difficulty, swelling or collapse is a medical emergency call 999. Never take, re-take or dose the medicine without professional advice.
Common triggers
- Codeine and products containing it
Symptoms
- Skin rash, hives or itching
- Swelling of the face, lips or throat
- Wheezing or difficulty breathing
- Dizziness or a drop in blood pressure
- In severe cases, anaphylaxis
Avoidance & management
- Tell every doctor, pharmacist and dentist about the allergy
- Wear a medical-alert bracelet if advised
- Check the ingredients of combination medicines
- Discuss safe alternatives with your prescriber
- Seek emergency care for a severe reaction
Sources & references
Allergy safety: what everyone should know
Mild-to-moderate reaction signs
- Itching, hives or a raised rash
- Swelling of lips, face or eyes
- Tingling mouth, sneezing, runny nose
- Stomach pain, nausea or vomiting
Anaphylaxis — call 999 (UK) or 112 (EU) immediately
- Difficult or noisy breathing, wheeze, persistent cough
- Swelling of the tongue or tightness in the throat
- Dizziness, collapse, pale and floppy (young children)
- Use an adrenaline auto-injector (e.g. EpiPen) at once if prescribed, then call an ambulance even if symptoms improve
Allergy or intolerance? A true allergy involves the immune system and can escalate quickly; an intolerance (e.g. lactose intolerance) is unpleasant but not life-threatening. The symptoms overlap, so self-diagnosis is unreliable.
Getting tested: home-testing kits based on IgG are not diagnostically valid. If you suspect an allergy, ask a GP for referral to an allergy clinic for skin-prick or specific-IgE testing and a supervised food challenge where appropriate.