Probiotics are living bacteria and yeasts that provide health benefits when consumed in large amounts.

They can be taken as supplements or consumed naturally through fermented foods like yogurt, kefir, sauerkraut, kimchi and kombucha (1, 2, 3, 4).

The health benefits of probiotic supplements and foods have been well documented, including a lower risk of infections, improved digestion and even a reduced risk for some chronic diseases (5, 6, 7, 8).

While there are many health benefits linked to taking probiotics, there can also be side effects. Most of these are minor and only affect a small percentage of the population.

However, some people with serious illnesses or compromised immune systems may experience more severe complications.

This article reviews the most common side effects of probiotics and how to reduce them.

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While most people do not experience side effects, the most commonly reported reaction to bacteria-based probiotic supplements is a temporary increase in gas and bloating (9).

Those taking yeast-based probiotics may experience constipation and increased thirst (10).

It is not known exactly why some people experience these side effects, but they typically subside after a few weeks of continued use (9).

To reduce the likelihood of side effects, start with a low dose of probiotics and slowly increase to the full dosage over a few weeks. This can help your body adjust to them.

If the gas, bloating or any other side effects continue for more than a few weeks, stop taking the probiotic and consult a medical professional.

Summary Some people experience an increase in gas, bloating, constipation or thirst when they start taking probiotics. These side effects should go away within a few weeks.

Some probiotic-rich foods, like yogurt, sauerkraut and kimchi, contain biogenic amines (11, 12).

Biogenic amines are substances that form when protein-containing foods age or are fermented by bacteria (13).

The most common amines found in probiotic-rich foods include histamine, tyramine, tryptamine and phenylethylamine (14).

Amines can excite the central nervous system, increase or decrease blood flow and may trigger headaches in people sensitive to the substance (15, 16).

One study found that low-histamine diets reduced headaches in 75% of participants. However, a review of 10 controlled studies found no significant effect of dietary amines on headaches (17, 18).

More research is needed to determine whether or not amines can be direct triggers of headaches or migraines in some people.

Keeping a food diary including any headache symptoms you might experience can help clarify whether fermented foods are problematic for you.

If probiotic-rich foods trigger your symptoms, a probiotic supplement may be a better choice.

Summary Fermented foods rich in probiotics naturally contain amines. Some people may experience headaches after eating these foods, and should instead opt for probiotic supplements.

Some bacterial strains used in probiotic supplements can produce histamine inside the digestive tract of humans (19, 20, 21).

Histamine is a molecule that is normally produced by your immune system when it detects a threat.

When histamine levels rise, blood vessels dilate to bring more blood to the affected area. The vessels also become more permeable so that immune cells can easily get into the relevant tissue to combat any pathogens (22).

This process creates redness and swelling in the affected area, and can also trigger allergy symptoms such as itching, watery eyes, runny nose or trouble breathing.

Normally, histamine that is produced in your digestive tract is naturally degraded by an enzyme called diamine oxidase (DAO). This enzyme inhibits histamine levels from rising enough to cause symptoms (23).

However, some people with histamine intolerance have trouble properly breaking down the histamine in their bodies, seeing as they do not produce enough DAO (24, 25, 26).

The excess histamine is then absorbed through the lining of the intestinal tract and into the bloodstream, causing symptoms similar to an allergic reaction (27).

People with a histamine intolerance should avoid foods that contain excess histamine (28).

Theoretically, they may want to select probiotic supplements that do not contain histamine-producing bacteria, but to date, there has been no research on this specific area.

Some histamine-producing probiotic strains include Lactobacillus buchneri, Lactobacillus helveticus, Lactobacillus hilgardii and Streptococcus thermophilus (29, 30, 31).

Summary Some probiotics can produce histamine within the digestive tract. Those with histamine intolerance may want to avoid these strains of bacteria.

People with allergies or intolerances should read the labels of probiotic supplements carefully, since they might contain ingredients they could react to.

For example, some of the supplements contain allergens such as dairy, egg or soy.

These ingredients should be avoided by anyone who is allergic, as they may trigger an allergic reaction. If necessary, read labels carefully to avoid these ingredients (32).

Similarly, yeast-based probiotics should not be taken by those with yeast allergies. Instead, a bacteria-based probiotic should be used (33).

Milk sugar, or lactose, is also used in many probiotic supplements (34).

While studies suggest that most people with lactose intolerance can tolerate up to 400 mg of lactose in medications or supplements, there have been case reports of adverse effects from probiotics (35, 36, 37).

Since a small number of people with lactose intolerance may experience unpleasant gas and bloating when consuming lactose-containing probiotics, they may want to choose lactose-free products.

In addition to containing powerful probiotics, some supplements also contain prebiotics. These are plant fibers that humans cannot digest, but that bacteria can consume as food. The most common types are lactulose, inulin and various oligosaccharides (38).

When a supplement contains both probiotic microorganisms and prebiotic fibers, it is called a synbiotic (39).

Some people experience gas and bloating when consuming synbiotics. Those who experience these side effects may want to select a supplement that does not contain prebiotics (40).

Summary Probiotic supplements can contain allergens, lactose or prebiotic fibers that may cause adverse reactions in some people. These ingredients can be avoided by reading labels.

Probiotics are safe for the vast majority of the population, but may not be the best fit for everyone.

In rare cases, the bacteria or yeasts found in probiotics can enter the bloodstream and cause infections in susceptible individuals (41, 42, 43, 44).

Those at greatest risk for infection from probiotics include people with suppressed immune systems, prolonged hospitalizations, venous catheters or those who have undergone recent surgeries (45, 46, 47).

However, the risk of developing an infection is very low, and no serious infections have been reported in clinical studies of the general population.

It is estimated that only about one in one million people who take probiotics containing Lactobacilli bacteria will develop an infection. The risk is even smaller for yeast-based probiotics, with only about one in 5.6 million users getting infected (48, 49).

When infections do occur, they typically respond well to traditional antibiotics or antifungals. However, in rare cases, deaths have occurred (48, 50).

Research also suggests that people with severe acute pancreatitis should not take probiotics, as this may increase the risk of death (51).

Summary People with compromised immune systems, venous catheters, recent surgery, acute pancreatitis or prolonged hospitalizations should avoid taking probiotics.

Probiotics are living microorganisms that provide health benefits when consumed in large quantities. They can be taken as supplements, but also occur naturally in fermented foods.

Probiotics are safe for the majority of the population, but side effects can occur. The most common side effects are a temporary increase in gas, bloating, constipation and thirst.

Some people can also react poorly to ingredients used in probiotic supplements or to naturally occurring amines in probiotic foods. If this occurs, stop using probiotics.

In rare cases, people with compromised immune systems, prolonged hospitalizations or recent surgeries may develop an infection from probiotic bacteria. People with these conditions should weigh the risks and benefits before consuming probiotics.

Overall, probiotics are a beneficial addition to most people’s diet or supplement regimen, with relatively few and unlikely side effects.