Probiotics, or “friendly bacteria,” are considered to be remarkably safe as nutritional supplements. Probiotic bacteria such as l. acidophilus have been used successfully to treat dozens of common ailments with very little risk of side effects, drug interactions or other unwanted results. However, like all nutritional supplements, there are some risks to taking large quantities of probiotics, and supplementation may not be right for all people.

Digestive Problems

Side effects from probiotics rarely occur, but the most common side effect is gastrointestinal distress. When large doses of probiotics are consumed, they adjust the floral balance of the digestive tract, possibly resulting in gas or abdominal discomfort. These side effects are usually temporary and, ultimately, benign.
In general, probiotics will have the opposite effect: Probiotic supplements are helpful in treating gastrointestinal disturbances such as bacterial infections rotoviruses and irritable bowel syndrome. The process of biological adjustment, not the probiotic supplements themselves, is probably the cause of the stomach problems.


It is theoretically possible that the live bacteria in probiotic supplements could overcolonize the intestines, causing infection. While there are no case reports of this complication occurring, it may be a risk for newborns, the elderly and people with severely compromised immune systems. Treatment for an infection of this nature may require antibiotics.
Because of this potential risk, people who are immunocompromised should exercise caution when taking very large doses of probiotics. However, it should be noted that the risk of this side effect is minimal. Probiotics are commonly consumed in foods and nutritional supplements by people of all ages and conditions, and no infection-related complications have been known to occur.


Overstimulation of the immune system is another complication that is theoretically possible but thus far unrecorded. If the body “mistakes” probiotics for foreign, infectious invaders, it may result in a response similar to infection. This can cause an elevated white blood cell count, fatigue and possibly even fever.
Probiotics have not been thoroughly studied in people with autoimmune diseases. Although they may be beneficial to people with autoimmune disorders, people in this group may be more at risk for this rare and unknown complication. Anyone with an autoimmune disorder or an unusually aggressive immune system should use caution when taking large doses of probiotics.

Metabolic Changes

Probiotics may slightly alter the function of the colon, resulting in unusual shifts in metabolism. Some people may experience more frequent bowel movements while taking probiotics, while others may experience a slow-down of bowel habits. Metabolic changes can result in weight gain, weight loss or malabsorption.
For most people who use probiotics, the adjustments in the body’s metabolism will be positive and desired. However, if these changes are uncomfortable or harmful to the health of the person taking the probiotics, they should adjust dosage or stop taking the supplements.

Avoiding Risks

Although probiotics are quite safe, there is risk associated with using them in place of conventional treatments. Self-diagnosis and self-treatment for mild, short-lived conditions can be useful, but no alternative remedy should be used in place of professional medical help, especially for serious or chronic diseases.
The few risks associated with probiotics may be greater in people who are very young, very old or suffering from a condition that seriously impairs the immune system. People in these categories should discuss probiotics with a qualified physician before taking them. Anyone experiencing unwanted side effects from probiotics should promptly report the problem to a medical professional.