The types of milk intolerance are distinguished mainly by their cause and at what point in the patient’s life they develop. All of these types of milk intolerance share the same symptoms and are treated in a similar manner. The types of milk intolerance include:

• Adult lactose intolerance. Symptoms usually begin in the teen years. This is the most common type of milk intolerance and is often an inherited condition. A small amount of milk may be tolerated each day (about 8 ounces on average), depending on an individual's production of lactase. The severity of this type of intolerance usually persists or slightly worsens as the individual grows older.
  
  
• Acquired lactose intolerance (secondary lactose intolerance). Occurs when the small intestine partially or completely discontinues the production of the enzyme lactase. This condition often comes as a result of an illness (e.g., viral stomach flu) or another medical condition (e.g. cystic fibrosis, celiac disease). This type of milk intolerance can occur at any age. Whether this condition is temporary or permanent depends on the amount of damage to the intestine and, therefore, the amount of lactase the body is able to produce.
  
  
• Congenital lactose intolerance. Rare but permanent condition in which children are born without the ability to produce lactase. Patients with this type of milk intolerance must avoid foods or drinks with lactose for their entire lives. 

Some factors increase the risk of an individual developing milk intolerance. These include:

• Family history. Milk intolerance has been linked to a specific genetic variation, and parents with an intolerance are more likely to pass this condition on to their children.
  
  
• Ethnicity. Certain ethnic and racial groups suffer from milk intolerance more than others. For instance, African Americans, Asian Americans and Native Americans are much more likely to have this condition than are individuals of northern European descent. Hispanics and those of Ashkenazi Jewish descent are also at increased risk.
  
  
• Premature birth. Because the lactase enzyme increases in the fetus during the third trimester, there is increased likelihood of infants developing milk intolerance if they are born at 28 to 32 weeks of gestation.
  
  
• Certain medications. People taking certain antibiotics risk developing temporary milk intolerance. This occurs when the medication interferes with the body's ability to produce lactase.
  
  
• Age. Most people become lactose intolerant over time. This occurs as the body ages and stops producing lactase.