Lactose intolerance is a condition marked by one’s inability to properly digest the lactose found in milk. The condition is often confused with a milk allergy (or casein allergy). The two conditions often appear quite similar. Likewise, they’re both set off when someone consumes dairy products. But lactose intolerance is a digestive issue while milk allergies are triggered by the immune system, and may cause an anaphylactic response.

Often, underlying signs of lactose intolerance are discovered by looking for the following signs:

1. Stomach pain

Stomach pain is one of the most striking and easily noticed signs of lactose intolerance. It’s primarily caused by lactose passing through the digestive system without being properly broken down. Our digestive system is where everything we eat is supposed to be deconstructed into the most basic nutritional components. People suffering from lactose intolerance can’t break down lactose on their own. They’ll typically feel pain as their stomach ramps up its stomach acid and lactose makes its way into systems, which aren’t able to handle undigested food. The pain might be further provoked by gas from other parts of the body making its way into the stomach.

2. Diarrhea or loose stools

Digestive problems can often result in fluid retention within the colon. As this fluid builds up it will eventually be ejected along with any remaining physical waste. This causes diarrhea. It’s often made even more severe when our internal microbiome partially digests lactose, which has made its way into the colon.

3. Increased gas or bloating

Our microbiome consists of helpful microorganisms that assist our bodies in a number of different ways. However, when the microbes in our colon encounter undigested lactose they’ll typically try to make a meal of it for themselves. This often results in waste products as the microbes digest the sugar in lactose. These byproducts will often cause swelling and gas. The microbiome is responsible for a surprisingly large number of symptoms associated with lactose intolerance. We find that internal bacteria are usually a huge boon to our lives. But they’re simply not very well equipped to deal with biologically useful but undigested food.

4. Constipation

This is the flip side of the earlier symptom of diarrhea. In this case people experience an inability to properly expel solid waste from their body. This symptom is usually a side effect of our microbiome’s waste products. If the microbiome produces enough methane while breaking down lactose it can impair our body’s ability to move food through our gut.

5. Headaches

A headache isn’t directly caused by lactose intolerance. However, they do come along with it in a large number of cases. The headaches typically manifest when a primary symptom causes enough internal distress. For example, people suffering from extreme diarrhea might experience a headache due to a combination of stress and dehydration. Technically, this is caused by diarrhea rather than lactose intolerance.

6. Mouth ulcers

Ulcers are often associated with lactose intolerance. This is one of the rarer symptoms, but also one of the most bothersome. At the moment researchers don’t even have a firm link between ulcers and lactose intolerance. The only solid fact we have is that someone with lactose intolerance will have a far greater likelihood of ulcers, or canker sores, in their mouth than random sampling would predict.