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Overview: Indole-3-carbinol (I3C) is a natural chemical found in some vegetables, such as broccoli, cabbage, cauliflower, and Brussels sprouts. It is also sold as a dietary supplement, usually as capsules or tablets. People often take I3C for cancer prevention and conditions linked to human papillomavirus (HPV), such as abnormal cervical cells. It is also taken to help balance estrogen levels. However, the benefits of I3C are not well defined. Before taking I3C, make sure to tell your health care provider about any health conditions you have or medicines you are taking.

Uses

What is indole-3-carbinol used for?

Indole-3-carbinol (I3C) is used for various reasons, but its benefits are not well defined. Some common reasons for using indole-3-carbinol include the following.

  • To lower the risk of certain cancers
  • To help with human papillomavirus (HPV)-related conditions, such as cervical dysplasia and warty growths in the airway (recurrent respiratory papillomatosis)
  • Hormone-related cancer prevention (such as breast cancer) and estrogen balance 
  • Antioxidant and anti-inflammatory support

Note: The FDA has not reviewed indole-3-carbinol for safety and effectiveness. Learn more about dietary supplements here.

How does indole-3-carbinol work (mechanism of action)?

Indole-3-carbinol (I3C) is a natural chemical found in cruciferous vegetables, such as broccoli, cabbage, cauliflower, Brussels sprouts, and radishes. In cruciferous vegetables, I3C forms from a compound called glucobrassicin when the vegetable is cut, chewed, or digested. Once I3C reaches the acid in your stomach, it changes into other chemicals. The main chemical is called 3,3'-diindolylmethane (DIM). Many of the effects seen in the body are thought to come from DIM and related products, not from I3C itself. In fact, studies in women found that I3C could not be measured in the blood after it was swallowed, and only DIM could be measured.

In laboratory and animal studies, I3C and DIM affect many pathways in cells. They can change how cells grow, divide, and die. They can also change the activity of liver enzymes that break down chemicals, hormones, and drugs.

One well-studied effect is on estrogen. I3C shifts the way the body breaks down estrogen toward a form that is thought to be less active. This is why I3C is often studied for hormone-related conditions. In studies in people, I3C raised the ratio of these estrogen breakdown products. However, I3C has not been proven to prevent cancer in people.

Some studies in people suggest I3C may help with HPV-related conditions, such as abnormal cervical cells and airway papillomas. But these studies are small and preliminary. The benefits of I3C are not well defined. Evidence in humans is limited. In animal studies, the timing of I3C has also mattered. In some cases, it blocked tumors, but in other cases, it seemed to promote them. Because of this, some experts advise caution with long-term use of I3C supplements for cancer prevention until more is known.

How should I store indole-3-carbinol?

You should store indole-3-carbinol according to the product label. Most dietary supplements should be kept in a cool, dry location that is out of the reach of children and pets and not in direct sunlight.

Side Effects

What are the most common side effects of indole-3-carbinol?

The most common side effects of indole-3-carbinol are rash, nausea, diarrhea, and other stomach problems. These side effects may be more likely with higher doses. Side effects may vary depending on the dose you are taking or the specific product you are using. Tell your health care provider if you have side effects that bother you.

There may be other side effects of indole-3-carbinol that are not listed here. Contact your health care provider if you think you are having a side effect of a supplement. In the U.S., you can report side effects to the FDA at www.safetyreporting.hhs.gov or by calling 888-SAFEFOOD (888-723-3366). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of indole-3-carbinol?

While less common, the most serious side effects of indole-3-carbinol are described below, along with what to do if they happen.

Severe Allergic Reactions. Indole-3-carbinol may cause allergic reactions, which can be serious. Stop taking indole-3-carbinol and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching or skin rash
  • Bumps on the skin called hives that can be red, pink, white, or brown depending on your skin tone
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Balance Problems and Shaking. High doses of I3C have been linked to problems with balance and shaking (tremor) in some people. These symptoms went away when the dose was lowered. Talk to your health care provider if you notice balance problems or shaking while taking I3C. Do not take more than the dose listed on the product label.

Warnings & Precautions

Who should not use indole-3-carbinol?

Allergies to Ingredients. People who are allergic to any of the following should not take indole-3-carbinol.

  • 3-indolylmethanol
  • I3C
  • Indole-3-carbinol
  • Indole-3-methanol
  • Any of the ingredients in the specific product

Your pharmacist can tell you all of the ingredients in the specific indole-3-carbinol products they stock.

Vegetable Allergies. People with a known allergy to cruciferous vegetables, such as broccoli, cabbage, or Brussels sprouts, should not take I3C without talking to their health care provider. I3C comes from these plants and could cause an allergic reaction.

What should I know about indole-3-carbinol before using it?

Indole-3-carbinol is available as a dietary supplement. It cannot be marketed to treat or cure a disease.

Indole-3-carbinol is available in many forms, including capsules and tablets. It is sometimes sold in combination with a related chemical called 3,3'-diindolylmethane (DIM). Read and follow the directions on the specific product you are taking, or take it as recommended by your health care provider.

I3C can change the way your body breaks down some medicines. If you take any prescription or over-the-counter (OTC) medicines, talk to your health care provider before using I3C. See the Interactions section for more details.

In animal studies, I3C has both blocked and, at times, promoted the growth of tumors, depending on when it was given. For this reason, some experts advise caution with the long-term use of I3C supplements for cancer prevention until we know more. If you are thinking about using I3C for cancer prevention or to treat a health condition, talk to your health care provider first.

Keep indole-3-carbinol out of the reach of children.

The FDA has not tested indole-3-carbinol products to confirm that they contain the ingredients stated on their labels. Some dietary supplements have been tested by third-party organizations to confirm that they contain the ingredients listed on their labels and do not contain any harmful chemicals. Learn more about how to choose a safe dietary supplement product here.

What should I tell my health care provider before using indole-3-carbinol?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. Also tell them if you have an upcoming surgery. Sharing this information will help your health care provider know if this or other supplements are right for you.

In particular, make sure that you discuss any of the following.

Hormone-Sensitive Cancer. I3C affects estrogen in your body. In animal studies, some data link I3C with tumor growth, while other data link it with blocking tumor growth. If you have a hormone-sensitive cancer, talk to your health care provider before taking I3C.

Liver Problems. I3C may change the activity of certain liver enzymes. Mild increases in liver enzymes have been reported in some people taking I3C. If you have liver problems, talk to your health care provider before taking I3C.

Other Medicines and Supplements. Before taking indole-3-carbinol, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how indole-3-carbinol could affect pregnancy or harm a fetus. Animal studies have shown mixed results, and I3C supplements are generally not recommended during pregnancy. Tell your health care provider if you are pregnant or plan to become pregnant.

Breastfeeding. It is not known if indole-3-carbinol passes into breast milk. Animal studies have shown mixed results, and I3C supplements are generally not recommended while breastfeeding. Tell your health care provider if you are breastfeeding or plan to breastfeed.

Interactions

Does indole-3-carbinol interact with foods or drinks?

There are no known interactions between indole-3-carbinol and foods or drinks. I3C is a chemical that is naturally found in cruciferous vegetables (such as broccoli and cabbage), so you may also get small amounts of it from food.

It is unknown if drinking alcohol will affect indole-3-carbinol.

Does indole-3-carbinol interact with medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using so they can watch for any potential interactions.

I3C can increase the activity of liver enzymes that break down many medicines, including one called CYP1A2. In theory, this could lower the levels of some medicines in your body and make them not work as well. If you take any medicines or supplements, talk to your health care provider before taking I3C so they can watch for problems.

It is not known whether I3C changes how estrogen medicines work. However, because I3C shifts how the body breaks down estrogen, tell your provider if you are using any product containing estrogen before taking I3C. Estrogen can be found in certain pills, patches, creams, rings, or vaginal suppositories.

The interactions between indole-3-carbinol and medicines are not fully understood. As with most dietary supplements, the research on drug interactions with indole-3-carbinol is incomplete. Always check with your health care provider before taking indole-3-carbinol with any medicines.

Overdose/Missed Dose

What should I do if I accidentally use too much indole-3-carbinol?

If you or someone else has used too much indole-3-carbinol, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

Indole-3-carbinol is usually sold as capsules or tablets. Read the product label closely, and speak with a health care provider about how to take indole-3-carbinol safely. Do not take double or extra doses, even if you miss a dose.