Milk Allergy or Lactose Intolerance? How Can I Tell the Difference in My Baby?

If your baby is struggling with diarrhoea, wind, tummy pain, reflux or feeding problems, it is easy to wonder whether they are lactose intolerant.

But in young babies, lactose intolerance and cow’s milk allergy are very different things, and they need different management.

Cow’s milk allergy is an immune reaction to the proteins in milk. Lactose intolerance happens when the body cannot digest lactose, the natural sugar found in milk.

The symptoms can overlap, which is why they are often confused.

The important thing is not to assume that your baby needs lactose free milk or to start removing foods without understanding what is causing the symptoms.

In this guide, I’ll explain the difference between milk allergy and lactose intolerance in babies, what symptoms to look for and when it is worth getting individual advice.

Milk allergy vs lactose intolerance in babies: the quick answer

	Cow’s milk allergy	Lactose intolerance
What causes it?	The immune system reacts to milk protein	The gut cannot digest lactose, the sugar in milk
Common in babies?	Yes, especially in infancy	Primary lactose intolerance is uncommon in young babies
Symptoms	Can affect the gut, skin and breathing	Mainly gut symptoms
Can symptoms be immediate?	Yes, with IgE-mediated allergy	Usually digestive symptoms after lactose is eaten
Can it be serious?	Sometimes	No, it does not cause anaphylaxis
Is lactose-free formula suitable?	No, if it still contains cow’s milk protein	Sometimes, if lactose intolerance is confirmed

What is cow’s milk allergy?

Cow’s milk allergy happens when your baby’s immune system reacts to proteins in cow’s milk, mainly casein and whey.

It is one of the most common food allergies in babies and young children. Symptoms can be immediate or delayed.

Immediate, or IgE-mediated, reactions usually happen within minutes and can include:

• Hives
• Swelling
• Vomiting
• Coughing or wheezing
• Breathing difficulty

Delayed, or non-IgE-mediated, reactions can be harder to recognise and may include:

• Persistent diarrhoea
• Constipation
• Blood or mucus in poo
• Vomiting or reflux
• Eczema
• Feeding difficulties
• Poor weight gain

One symptom on its own does not confirm cow’s milk allergy. The pattern, timing and overall history matter.

If you are wondering whether your baby’s symptoms could be CMPA, you can also read my guide to

What is lactose intolerance?

Lactose intolerance is not an allergy.

It happens when the gut does not have enough of an enzyme called lactase to break down lactose, the natural sugar found in milk.

The symptoms are mainly digestive and can include:

• Diarrhoea
• Bloating
• Wind
• Tummy pain
• Gurgling
• Acidic stools that can irritate the skin around the bottom

Lactose intolerance does not cause hives, swelling, wheezing or anaphylaxis because it does not involve the immune system.

Is lactose intolerance common in babies?

This is where there is often a lot of confusion.

Primary lactose intolerance is very uncommon in young babies.

Babies are naturally designed to digest lactose because lactose is the main carbohydrate in breast milk.

Primary lactose intolerance usually develops later in childhood or adulthood as lactase levels naturally reduce. Allergy UK specifically notes that lactose intolerance is uncommon in babies and children under three.

So if a young baby has ongoing symptoms such as reflux, eczema, blood in their poo or poor growth, I would not automatically assume lactose intolerance.

Can babies get temporary lactose intolerance?

Yes.

Babies can sometimes develop secondary lactose intolerance after a gut infection or other inflammation affecting the gut lining.

For example, after gastroenteritis (tummy bug), the gut may temporarily produce less lactase.

This can cause:

• Watery diarrhoea
• Increased wind
• Bloating
• Acidic stools
• Sore skin around the bottom

This type of lactose intolerance is usually temporary and improves as the gut heals.

That is very different from primary lactose intolerance and also different from cow’s milk allergy.

What symptoms are more suggestive of cow’s milk allergy?

Cow’s milk allergy is more likely to be considered when symptoms involve more than just the gut.

For example:

• Eczema alongside digestive symptoms
• Hives or swelling
• Wheezing or coughing
• Blood or mucus in the poo
• Persistent vomiting
• Feeding difficulties
• Faltering growth
• Symptoms that repeatedly occur after exposure to cow’s milk protein

Cow’s milk allergy can affect the gut, skin, respiratory system and, in severe cases, circulation.

Lactose intolerance mainly causes digestive symptoms.

Can lactose free formula help cow’s milk allergy?

No.

Lactose free formula can still contain cow’s milk protein.

That means it is not suitable for treating cow’s milk allergy.

The NHS advises that babies with diagnosed cow’s milk allergy may need an appropriate hypoallergenic formula under medical supervision, rather than a standard lactose-free formula.

If your baby’s symptoms improved on a lactose free formula, it does not automatically prove that they were lactose intolerant.

There may be other reasons symptoms changed, which is why it is important to look at the full history rather than relying on one formula trial.

If you are unsure which formula your baby needs, my guide below can help.

Can a baby have both milk allergy and lactose intolerance?

Yes, but this is less common.

A baby with cow’s milk allergy can occasionally develop temporary secondary lactose intolerance if the gut lining has become inflamed.

This is one reason symptoms can sometimes be confusing.

But the two conditions are still different.

One is an immune reaction to milk protein, and the other is difficulty digesting milk sugar.

This is also why the right treatment depends on what is actually causing the symptoms.

Should I remove milk from my baby’s diet to see what happens?

I would not recommend removing a major food group or changing formula without a clear reason or professional advice.

Milk and dairy foods are important sources of energy, protein, calcium, iodine and other nutrients for babies and young children. The NHS advises speaking to a GP or dietitian before removing them unnecessarily.

If cow’s milk allergy is suspected, diagnosis usually involves looking carefully at:

• The symptoms
• Timing
• Feeding history
• Growth
• Response to removing cow’s milk protein
• and, where appropriate, planned reintroduction

For immediate reactions, allergy testing may also be useful.

The process is different from simply switching to lactose-free foods.

What if I’m still not sure which one my baby has?

This is one of the areas where the pattern matters much more than one symptom.

I would look at:

• How old your baby is
• When the symptoms started
• Whether there has been a recent tummy bug
• Whether symptoms affect just the gut or other systems too
• Whether your baby has eczema
• Whether there is blood or mucus in the poo
• What milk or formula they are having
• Whether symptoms happen consistently after cow’s milk exposure
• Growth and feeding

That is often enough to start building a much clearer picture.

If you are unsure whether your baby’s symptoms fit cow’s milk allergy, lactose intolerance or something else, I can help you work through the history and decide what the next step should be.

When should I seek urgent medical help?

Lactose intolerance does not cause anaphylaxis.

Cow’s milk allergy can sometimes cause an immediate allergic reaction.

Seek urgent medical help if your baby develops:

• Difficulty breathing
• Swelling of the tongue or throat
• Significant facial swelling
• Becomes floppy or very unwell
• Signs of anaphylaxis

If you think your baby is having a severe allergic reaction or anaphylaxis, call 999 for emergency medical help