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Other common name(s): lactic acid bacteria

Scientific/medical name(s): Lactobacillus acidophilus (L. acidophilus)


Acidophilus is a type of germ or bacterium commonly found in the normal digestive tract of mammals, mainly in the small intestine. It is also found in many dairy products, especially yogurt. Acidophilus and some related bacteria are considered to be "probiotic" because they may help the body maintain or restore its normal balance of helpful bacteria.


Acidophilus has been promoted for a wide variety of conditions, including cancer. Animal studies looking at the role of L. acidophilus in reducing the risk of cancer have shown varying results. There have been no studies with humans on the role of Lactobacillus acidophilus in preventing or treating human cancers.

How is it promoted for use?

Acidophilus is often promoted as a supplement to help “maintain bowel health.” It has also been suggested to prevent or treat diarrhea and vaginal infections, to lower cholesterol, to help with lactose digestion in lactose-sensitive people, and to help prevent the growth of the types of bacteria and yeast that can cause illness.

Some supporters claim acidophilus may lower the risk of cancer, especially colon cancer. It is supposed to do this by neutralizing cancer-causing agents (carcinogens) in the diet and by directly killing tumor cells. Some also claim that acidophilus works against cancer by boosting the immune system by making B vitamins and vitamin K, and that it reduces cholesterol levels, which proponents say tumor cells need in order to grow.

What does it involve?

When taking acidophilus, the dosage usually refers to the number of live bacteria. Most sources suggest 1 to 15 billion bacteria as a recommended dose, although some studies have used larger amounts. It’s available in tablet, capsule, liquid, and powder form. Average dosage suggestions vary from 1 to 3 times per day. However, some scientists warn that the concentration of the bacteria in the supplements varies widely from one manufacturer to another. Yogurt with live cultures and milk with L. acidophilus added to it are other sources. Some women treat vaginal problems by taking acidophilus, either by mouth or by putting liquids containing acidophilus into the vagina. A few recommend inserting whole tablets into the vagina, even those that are not designed for use in the vagina.

What is the history behind it?

Interest in the health benefits of acidophilus began in the late 1800s when it was proposed that the long life span of the Balkan people was due to their ingestion of fermented milk products. It was later found that these milk products were rich in L. acidophilus. Since then, the exact role of L. acidophilus in the digestive tract and in human health has been a subject of interest.

What is the evidence?

Laboratory and animal studies on the ability of L. acidophilus to prevent cancer have had mixed results, and there have been no large studies reported in humans. L. acidophilus has been studied in the laboratory for possible anti-tumor properties. In some studies, milk that was fermented by L. acidophilus was able to slow or prevent the growth of breast and colon cancer cells grown in the laboratory.

In other studies, animals that were given L. acidophilus were found to be less prone to DNA damage in the colon after being given known carcinogens, suggesting acidophilus might have an effect on colon cancer. However, animal studies have shown that diets that include L. acidophilus do not seem to affect the formation of breast or skin cancers. In either case, randomized studies in people have not been done. Further studies are needed to determine whether the results apply to humans.

Researchers have also studied the effects of L. acidophilus and other probiotics on certain reproductive hormones known in high levels to increase risk of breast cancer. In studies of both pre- and post-menopausal women, L. acidophilus had no effect.

A Japanese study looked at the effect of a related bacterium (Lactobacillus casei) on the risk of colon tumors in about four hundred men and women who had previous tumors removed. The risk of new tumor development was not significantly lower in those who took L. casei, although the tumors that did develop contained cells that were less abnormal.

A review of research on the effects of acidophilus and other closely related bacteria found that they lowered cholesterol in early studies, but later, better-controlled studies have not been consistent in showing this effect. It also found that a related bacterium (Lactobacillus GG) may shorten the duration of diarrhea due to viral or bacterial infections.

A 2010 Chinese study looking at hospitalized patients gave them doses of 50 billion, 100 billion bacteria (L. acidophilus plus L. casei), or placebo each day during antibiotic treatment and for 5 days after. The researchers reported that both Lactobacillus groups had less diarrhea from taking antibiotics than the placebo group, and the higher dose seemed to work better.

Finally, a 2012 analysis of 63 human studies looking at probiotic bacteria found that taking them reduced the risk of diarrhea from taking antibiotics. This review included studies of a number of different types of bacteria, although most used Lactobacillus species.

A few recent studies suggest that acidophilus or related Lactobacillus species may reduce the severity of diarrhea occurring with chemotherapy for colorectal cancer or with radiation therapy for colorectal or cervical cancer.

Are there any possible problems or complications?

This product is sold as a food or dietary supplement in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

There appear to be few short-term problems with taking acidophilus. Some people have reported excess bloating or gas for the first few days while taking the supplement. In rare cases, acidophilus may cause serious infections that are hard to treat with antibiotics. People with weakened immune systems, such as those who are taking steroids or undergoing chemotherapy, who have received organ transplants, or who have AIDS, should use acidophilus and other probiotics with caution.

The lack of standardization makes it hard to be sure of the quality of acidophilus and other probiotic products. Because acidophilus must contain live cultures in order to be effective, proper packaging and storage is essential. Many products may contain other bacteria or may not contain enough of the active organisms, especially if the product has been sitting on a shelf for a while.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

To learn more

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-ACS-2345).

Dietary Supplements: What Is Safe?

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods and Cancer

Placebo Effect

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 01/15/2013
Last Revised: 01/15/2013