Xylitol, a sugar alcohol that occurs naturally in many different plants and is often used as a sugar substitute, it typically well-tolerated by people in small amounts but can cause some unpleasant side effects when consumed in large quantities. The most common problem it causes is gastrointestinal upset, which can include bloating, gas, and diarrhea. It has been found to cause increases in uric acid levels in the blood, and can also cause low blood sugar levels. Other xylitol side effects can include a rash, swelling in the mouth or throat, and difficulty breathing if there is an allergic reaction to it.

Some of the main xylitol side effects that can occur are problems with the digestive system. Xylitol has laxative properties, so it can lead to stomach and intestinal upset, particularly in people who eat too much of it. Many people experience excessive gas in these cases, leading to discomfort, bloating, and flatulence. It may also lead to diarrhea if the person's normal digestive function is disrupted. One way to avoid these issues is to increase one's intake of xylitol slowly over time.

Another of the xylitol side effects that has been reported with high intake of the substance is an increase in uric acid levels. This can be a problem particularly for people who are given xylitol by IV in large quantities, which may be done to provide a source of energy to some patients. A high uric acid level may be an issue as it can increase the risk that a person will develop kidney stones.

Hypoglycemia is also one of the xylitol side effects that can occur in heavy consumers. Used as a sweetener to replace sugar, it can be very helpful in the diets of patients like diabetics who need to control their sugar intake. For others, however, too much of it may decrease their sugar intake to the point that blood sugar levels drop too low.

Some people may be allergic to xylitol, in which case they can experience side effects related to that reaction. They may feel itchy or get a rash or hives on their skin. Swelling may occur in the mouth or throat, making it difficult to swallow. In severe cases, the person may begin wheezing and have trouble breathing. This type of reaction can be very dangerous, so it is important to seek medical attention if it occurs.