Updated September 2026 to reflect current guidance and improve information on CMPA symptoms, diagnosis and when to seek help.
Cow’s milk protein allergy (CMPA) is one of the most common food allergies in babies, but the symptoms aren’t always easy to spot. Things like reflux, changes in poo, eczema, being unsettled or having feeding problems can all happen with CMPA — but they can also happen in babies who don’t have a milk allergy.
That’s why it’s really important to look at the whole picture rather than one symptom on its own. When did the symptoms start? Do they happen after milk? Are there symptoms affecting more than one area, such as the skin and tummy? And do things improve when cow’s milk protein is removed?
In this guide, I’ll take you through the most common CMPA symptoms in babies, explain the difference between immediate and delayed milk allergy, and help you understand what to do next if you’re worried your baby may have CMPA.
IgE vs non-IgE CMPA: What’s the difference?
There are two main types of cows milk allergy; IgE and non-IgE. The biggest difference parents tend to notice is how quickly the symptoms appear.
With an IgE milk allergy, symptoms usually happen quite quickly, often within minutes and generally within two hours of having cow’s milk protein. You might see things like hives, swelling, vomiting, coughing or wheezing.
Non-IgE milk allergy is more delayed. Symptoms can appear several hours later, which is one reason it can be much harder to recognise. These babies are more likely to have symptoms such as ongoing tummy problems, changes in poo or eczema.
Both are genuine food allergies. Non-IgE CMPA isn’t the same as lactose intolerance, even though the two are sometimes confused.
What are the symptoms of CMPA?
There isn’t one single symptom that tells us a baby definitely has cow’s milk protein allergy. Reflux, eczema, constipation, loose poo, crying and feeding difficulties are all common in babies, which is why CMPA can sometimes be difficult to recognise.
What I look for is the overall pattern of symptoms rather than one symptom on its own.
Things that may make me more suspicious of CMPA include:
- symptoms happening repeatedly after exposure to cow’s milk protein
- symptoms affecting more than one area of the body, such as the skin and tummy
- blood or mucus in the poo alongside other symptoms
- eczema that is difficult to manage
- persistent feeding difficulties or significant discomfort
- symptoms improving when cow’s milk protein is removed and returning when it is reintroduced
The timing of symptoms is also really useful. IgE reactions usually happen quickly, while non-IgE reactions can be delayed by several hours or longer.
Worried your baby might have CMPA?
If you are struggling to work out whether your baby’s symptoms fit with milk allergy, I offer online consultations for families who need individual support.
I can help you look at your baby’s symptoms and feeding history, understand what the next steps should be and make sure any dietary changes are nutritionally appropriate.
Not quite ready to book yet? Then you can download my symptom tracker
Symptoms of CMPA in Infants: IgE vs Non-IgE
The timing of symptoms can help identify whether a reaction is more likely to be IgE-mediated or non-IgE-mediated. IgE reactions usually happen quickly, while non-IgE reactions are delayed.
The table below shows the main differences
| IgE Milk Allergy Symptoms | Non IgE Milk Allergy Symptoms |
| An immediate reaction within minutes and up to 2 hours after having milk protein. | A delayed reaction which happens between 2-72 hours after having milk protein. |
| Swelling to lips, face or eyes | / |
| Skin reaction – hives, raised, itchy rash (urticaria), reddening. | Skin reaction – Itchy skin, reddening ‘erythema’, atopic eczema. |
| Gastrointestinal upset – nausea, vomiting, diarrhoea, tummy pain or discomfort, | Gastrointestinal upset – Reflux, vomiting, diarrhoea, blood and/or mucous in stools, constipation, tummy pain or discomfort, irritability. |
| Respiratory – cough, chest tightness, wheezing and shortness of breath. Nasal itching, sneezing, congestion, conjunctivitis. | Respiratory – Cough, chest tightness, wheezing and shortness of breath. |
CMPA Rash – What can it look like in babies?
Skin symptoms can happen with both IgE and non-IgE milk allergy, but they can look a little different.
With an IgE milk allergy, skin symptoms usually appear quickly after cow’s milk protein is eaten. You may see hives, redness or swelling around the lips, face or eyes.
With non-IgE CMPA, skin symptoms are more likely to be delayed. Some babies may have eczema that becomes more difficult to manage, or dry, itchy skin that seems to flare alongside other symptoms.
A rash on its own does not confirm CMPA, as babies can develop rashes and eczema for lots of different reasons. What is more helpful is the pattern — when the rash appears, whether it happens after milk exposure and whether there are other symptoms at the same time.
If your baby develops swelling of the tongue or throat, difficulty breathing, becomes floppy or unusually unresponsive, call 999/911.
CMPA Poo – what can it look like?
Changes in poo can happen in babies with cow’s milk protein allergy, but there isn’t one type of “CMPA poo” that confirms a milk allergy.
Some babies may have:
- mucus in the poo
- small streaks of blood
- very loose stools or diarrhoea
- constipation
- changes in how often they poo
- green or unusual-looking stools alongside other symptoms
Poo colour on its own is not enough to diagnose CMPA. Green, yellow and orange stools can all happen in babies for lots of different reasons.
What is more helpful is the overall pattern. For example, are the poo changes happening alongside reflux, eczema, feeding difficulties or other symptoms? Do they improve when cow’s milk protein is removed and return when it is reintroduced?
If you notice blood in your baby’s poo, particularly if it keeps happening or your baby seems unwell, speak to your GP or healthcare professional.
Example of mucus in a baby’s stool during assessment for possible cow’s milk protein allergy. Stool appearance alone cannot confirm CMPA.
Can CMPA Cause Breathing Problems in Babies?
Some babies with cow’s milk protein allergy can have breathing or nasal symptoms, usually as part of an IgE-mediated reaction.
These may include:
- coughing
- wheezing
- noisy or difficult breathing
- sneezing or a runny nose
- swelling around the lips, face or throat
Breathing symptoms are less common than tummy or skin symptoms, but they are more concerning because they can be part of a serious allergic reaction.
If your baby has difficulty breathing, swelling of the tongue or throat, becomes floppy or unusually unresponsive, or you are worried they may be having anaphylaxis, call 999/911.
If your baby regularly has a blocked or runny nose on its own, this does not necessarily mean they have CMPA. It is the timing and the combination of symptoms that matter most.
When do milk allergy symptoms start in babies?
Milk allergy symptoms can start quite early, sometimes within the first few weeks of life.
For some babies, symptoms become noticeable after they start having formula containing cow’s milk protein. For breastfed babies, the signs can be a little harder to spot because exposure to cow’s milk protein is much smaller.
You might also notice symptoms when your baby starts having formula top-ups or when dairy is introduced during weaning.
There isn’t a particular age when CMPA has to start. What matters more is the pattern of symptoms and whether they seem to be linked to cow’s milk protein.
If you’re unsure, keeping a record of symptoms, feeds and timing can be really helpful when you speak to your GP, health visitor or dietitian.
What causes CMPA in babies??
CMPA happens when a baby’s immune system reacts to proteins found in cow’s milk.
We don’t always know exactly why one baby develops a milk allergy and another doesn’t. A family history of allergic conditions such as eczema, asthma, hay fever or food allergy can increase the chance, but CMPA can also happen in babies with no obvious family history.
Both breastfed and formula-fed babies can develop CMPA. The difference is simply how they are exposed to cow’s milk protein.
In formula-fed babies, the milk protein comes directly from the formula. In breastfed babies, very small amounts of cow’s milk protein from the mother’s diet can pass into breast milk.
How is CMPA diagnosed?
There isn’t one single test that diagnoses every type of cow’s milk allergy. The way CMPA is diagnosed depends on whether your baby’s symptoms suggest an immediate IgE-mediated allergy or a delayed non-IgE allergy.
For non-IgE CMPA, blood tests and skin prick tests are not helpful. Diagnosis is usually based on your baby’s symptoms, a detailed allergy history and a short period of removing cow’s milk protein, followed by a planned reintroduction to see whether symptoms return. NICE recommends elimination followed by reintroduction when non-IgE food allergy is suspected.
This is important because lots of common baby symptoms can improve with time anyway. If milk is removed but never reintroduced, it can be difficult to know whether CMPA was actually the cause.
If your baby’s symptoms suggest an IgE-mediated milk allergy, your GP or allergy team may arrange a skin prick test or specific IgE blood test. These tests can support the diagnosis, but the results always need to be considered alongside your baby’s symptoms and allergy history.
Formula-fed babies
If your baby is formula-fed and non-IgE CMPA is suspected, your healthcare professional may recommend a trial of an extensively hydrolysed formula. If symptoms are severe or do not improve as expected, an amino acid formula may sometimes be needed.
After the elimination period, cow’s milk protein is usually reintroduced in a planned way to see whether symptoms return. This helps confirm whether CMPA is actually the cause.
Breastfed baby
If your baby is exclusively breastfed and CMPA is suspected, you may be advised to remove cow’s milk from your own diet for a short period while continuing to breastfeed.
Cow’s milk is then reintroduced into your diet in a planned way to see whether your baby’s symptoms return. It’s important that any maternal dairy-free diet is nutritionally balanced, particularly for calcium, iodine and vitamin D.
You can read more about this in my CMPA Milk Challenge guide.
Important: If your baby has had an immediate or severe reaction to milk, do not reintroduce cow’s milk at home unless you have been advised to do so by your allergy team.
Need help working out whether your baby has CMPA?
If you’re unsure whether your baby’s symptoms fit with milk allergy, or you’re struggling with elimination and reintroduction, I can help you work through the symptoms, feeding history and next steps.
Milk Allergy Tests for IgE-Mediated CMPA
If your baby has symptoms that happen quickly after having cow’s milk protein — for example hives, swelling, vomiting, coughing or wheezing — your healthcare professional may suspect an IgE-mediated milk allergy.
In this situation, they may arrange a skin prick test, a blood test, or sometimes both.
These tests can help support the diagnosis, but they don’t diagnose milk allergy on their own. The results always need to be looked at alongside your baby’s symptoms and allergy history.
Skin Prick Testing
A skin prick test checks for an immediate allergic response to cow’s milk protein.
A small amount of milk allergen is placed on the skin, usually on the forearm or back, and the skin is gently pricked. If your baby is sensitised to cow’s milk, a small raised bump may appear within around 15–20 minutes.
The size of the reaction can give useful information, but it does not tell us how severe a future allergic reaction might be.
Skin prick testing will take place in the hospital.
Blood Test for Milk Allergy in Babies
A specific IgE blood test measures antibodies to cow’s milk protein in your baby’s blood.
A positive result can support a diagnosis of IgE-mediated milk allergy, but it still needs to be interpreted alongside your baby’s symptoms.
A positive test does not always mean your baby will react when they eat milk, and a negative test does not always completely rule milk allergy out.
If non-IgE CMPA is suspected, skin prick tests and blood tests are not useful for diagnosis.
If your baby’s tests are negative, or their symptoms are delayed, that doesn’t automatically mean milk is not the problem. Non-IgE CMPA does not show up on skin prick or blood tests, which is why the full symptom history and response to elimination and reintroduction are so important.
Another condition that is often confused with CMPA is lactose intolerance, but the two are quite different.
Milk allergy or lactose intolerance – what’s the difference?
Cow’s milk protein allergy and lactose intolerance are often confused, but they are very different.
CMPA is an immune reaction to the proteins in cow’s milk. It can cause symptoms affecting the tummy, skin or breathing, depending on the type of allergy.
Lactose intolerance is different. It happens when the body has difficulty digesting lactose, the natural sugar in milk. It mainly causes tummy symptoms such as bloating, wind, diarrhoea and discomfort.
Lactose intolerance does not involve the immune system, so it does not cause symptoms such as hives, swelling or breathing problems.
True lactose intolerance is also uncommon in young babies.
Reflux, colic or CMPA?
Reflux, crying and colic are very common in babies and can also happen alongside CMPA, which is one reason milk allergy can be difficult to spot.
What makes me more suspicious of CMPA is when these symptoms happen alongside other signs, such as eczema, blood or mucus in the poo, feeding difficulties or a clear pattern linked to cow’s milk protein.
Reflux or colic on their own do not confirm CMPA, and improvement with treatment does not automatically rule it out either.
Not sure what’s causing your baby’s symptoms?
My free CMPA symptom tracker can help you keep a record of what your baby is experiencing, when symptoms happen and any patterns you notice.
This can be really useful when you speak to your GP, health visitor or dietitian.
Need individual help with possible CMPA?
If you’re still unsure whether your baby’s symptoms fit with cow’s milk allergy, I can help you work through the symptoms, feeding history and next steps.
Hannah Whittaker RD is a specialist paediatric allergy dietitian with over 20 years’ experience in nutrition and 10 years as a Registered Dietitian. She specialises in food allergy in babies and children, including cow’s milk, egg, soya, wheat, peanut and tree nut allergy, food reintroduction and nutritional adequacy.
Hannah has worked in NHS paediatrics and now supports families with practical, evidence-based nutrition advice. She is also an experienced media spokesperson and has contributed to publications including The Sunday Times, The Independent and Huffington Post.
Credentials
Registered Dietitian
First Class Degree in Nutrition
Postgraduate qualification in Dietetics
Experience
Over 20 years’ experience in nutrition
10 years as a Registered Dietitian
NHS paediatric dietetics experience
Media spokesperson in the UK and internationally
Writer, committee member and researcher supporting the British Dietetic Association
