With an estimated 65 to 70% of people in the world experiencing some degree of this condition, lactose intolerance is one of the most widely distributed ailments affecting people today. Not to be confused with a milk allergy, lactose intolerance is the body’s ability or lack thereof to process and metabolize lactose, a sugar found within milk or milk products. 
Alternatively, a milk (or casein) allergy is a bodily reaction to the proteins found in milk and can be much more harmful, and even fatal. Lactose intolerance is far less severe, consisting of mainly digestive symptoms. Let’s explore the most common symptoms of lactose intolerance in more detail

1. Stomach pain and cramps
It’s a safe bet to assume you might be lactose intolerant if, after a meal with milk or dairy products, you experience stomach pains or cramping. The inability of the body to process the lactose can cause distension, among other things, much of those can cause digestive distress. It is important to note, however, that the severity of pain depends wholly on the individual and their particular sensitivity to lactose

2. Diarrhea or loose stools
As your body struggles to ferment the lactose sugars in your colon, it may produce too much water or fluids in your system. This can lead to loose stools or diarrhea. The microflora of your gut might tend to reabsorb the short-chain fatty acids they produced during the process of digesting lactose, resulting in more frequent loose bowel movements

3. Gas and bloating
As stated in the previous paragraph, the microflora in your gut ferments the lactose when it reaches your large intestine. When it struggles to absorb the lactose, it’ll settle in and be broken down by the bacteria dwelling with it. This may lead to distention of the belly from the production of excess gas. Due to the nature of lactose intolerance, gas released by the gut is composed mainly of methane, hydrogen, and carbon dioxide. The amount of gas is usually an indicator of how efficiently the colon is absorbing the lactose in your system, as well as the production rate of the gas in question

4. Constipation
While diarrhea might be a marker of lactose intolerance, the presence of constipation might be a symptom for others. The cause of this is less common, but the release of methane when short-chain fatty acids are reabsorbed can cause the inability to pass or cause hardening of stools in some people

5. Mouth ulcers
One symptom that might be less known is mouth ulcers, also known as canker sores. These tend to manifest as small, stinging inflamed spots in the mouth, and are not related to cold sores, nor are they infectious or contagious. The presence of milk protein in the mouth could cause stress in the cheeks, gums, or even tongue of the person with the intolerance. It is important to note that oral health can deteriorate without the presence of calcium in the body, so if lactose intolerance is something you suspect, it’s still important to absorb calcium from other sources (i.e., fish with bones, fortified soy, curly kale, okra, etc.)

There are still more symptoms associated with this condition (i.e., headaches, fatigue, etc.), but the most common tend to be the ones listed above. Always consult your physician if you believe congruent symptoms could mean something like lactose intolerance.