Milk allergy vs. lactose intolerance: What's the difference?

Milk allergy and lactose intolerance are not the same thing. In fact, lactose intolerance and milk allergies actually aren't related at all. If you or someone you know has trouble consuming milk, it is important to understand the difference between an intolerance and an allergy.

A milk allergy is caused by the immune system reacting to one or both of the main proteins in milk: casein and whey. When someone with a milk allergy consumes milk or a product containing milk, the body attempts to fight it with an immune response, which results in the symptoms of an allergic reaction. There are different risk factors for milk allergy, including family history, age (it's more common in young children), history of other allergies and having atopic dermatitis.

Lactose intolerance is the inability to fully digest lactose, or milk sugar. It is most often caused by producing too little of an enzyme in the small intestine called lactase. This enzyme helps break down lactose so that it can be easily absorbed into the bloodstream. Without enough lactase, the undigested lactose passes into the colon where it’s fermented by bacteria, causing symptoms, such as diarrhea and bloating.

Lactose intolerance can develop over time as lactase production decreases with age. It can also result from illness, injury, or surgery involving the small intestine. Premature infants sometimes have lactose intolerance, but this is often a short-term problem that goes away. In rare cases, babies may be born with a genetic defect, which may make them unable to make any of the lactase enzyme.

Symptoms of a milk allergy, or any allergy, can be dangerous. Depending on the body's immune response, there can be immediate and delayed symptoms.

Immediate milk allergy symptoms may include:

• Hives
• Wheezing
  
• Itching around lips or mouth
• Coughing
• Shortness of breath
• Vomiting 

Delayed milk allergy symptoms may include:

• Loose stools or diarrhea, which may contain blood
• Abdominal cramps
• Runny nose
  
• Watery eyes
• Colic in babies

Most importantly, watch out for signs of anaphylaxis, which is a life-threatening allergic reaction.

Some of the symptoms of anaphylaxis include: 

• Increased difficulty breathing
• Constriction of airways
• Swollen throat
• Facial flushing
  
• Itching

Symptoms of lactose intolerance normally begin anywhere from 30 minutes to two hours after lactose consumption. These symptoms, while unpleasant, are never life-threatening. They tend to be gastrointestinal in nature and may include:

• Abdominal cramps or pain
• Nausea
• Diarrhea
  
• Bloating
• Gas

The only way to prevent an allergic reaction to milk is to avoid milk and milk-containing products. An over-the-counter antihistamine may be able to help with a mild allergic reaction. For an anaphylactic reaction, the treatment is a rescue medication called epinephrine and emergency medical care. People with a milk allergy may be prescribed an epinephrine auto-injector (EpiPen, Adrenaclick, others) to carry with them at all times.

Prevention and treatment for lactose intolerance depend on the cause and severity of the intolerance. If the lactose intolerance is caused by an underlying injury or infection, treating the issue may help. Otherwise, the most common route of easing symptoms of lactose intolerance  is to consume less lactose. Some over-the-counter lactose intolerance medications, such as Lactaid, contain the lactase enzyme and may help digest food containing lactose.

If you have symptoms when consuming milk or lactose, it's always best to let your healthcare provider know. They can evaluate you and advise you on lifestyle changes and medication. For a milk allergy, your provider can schedule appropriate tests and prescribe rescue medications, if needed. In the case of life-threatening symptoms, you should always seek immediate medical attention.

Updated by Amy Magill, MA, RDN, September 2024.