8 mins read

Overview: Knotweed is the common name for two related plants used in dietary supplements. The most common one is Japanese knotweed (Polygonum cuspidatum), which is a leading natural source of resveratrol. A related plant, common knotweed or knotgrass (Polygonum aviculare), is used differently and contains very little resveratrol. People take Japanese knotweed for antioxidant and anti-inflammatory support and to help manage blood sugar, cholesterol, and blood pressure, while common knotweed has traditionally been used for urinary problems, such as kidney stones, and for digestive complaints. The benefits of both plants for most uses are not well defined. Common side effects include stomach pain, nausea, and diarrhea, especially with higher doses. Before taking knotweed, make sure to tell your health care provider about any health conditions you have or medicines you are taking.

Uses

What is knotweed used for?

The name knotweed can refer to two related plants that are used in different ways. Most knotweed supplements are made from Japanese knotweed (Polygonum cuspidatum), which is commonly used for the following reasons. However, the benefits for most uses are not well defined.

Antioxidant and Anti-Inflammatory Support. Knotweed and its natural chemical resveratrol are commonly taken for general antioxidant and anti-inflammatory support.

Blood Sugar, Cholesterol, and Blood Pressure Support. Knotweed is also used by people with type 2 diabetes or metabolic syndrome to help manage blood sugar, cholesterol, and blood pressure.

A different plant called common knotweed or knotgrass (Polygonum aviculare) is sometimes sold under the knotweed name but is used in different ways. Its benefits are not well defined. Common knotweed has a long history of traditional use for the following.

Urinary and Kidney Health. Common knotweed has traditionally been used for urinary problems, including kidney stones, and to increase urine flow.

Digestive Complaints. It has traditionally been used for stomach and intestinal problems, such as ulcers, diarrhea, and hemorrhoids.

Heart and Metabolic Health. It has traditionally been used, including in Korean folk medicine, for concerns such as obesity and high blood pressure.

Do not use knotweed in place of prescribed diabetes, cholesterol, or blood pressure medicines. Talk to your health care provider about your options or if you have concerns.

Note: The FDA has not reviewed knotweed for safety and effectiveness. Learn more about dietary supplements here.

How does knotweed work (mechanism of action)?

Knotweed (Polygonum cuspidatum) is a plant in the buckwheat family. It is also known by the scientific names Reynoutria japonica and Fallopia japonica and by the traditional Chinese medicine name Hu Zhang. The root (rhizome) of the plant is the part used in supplements. Knotweed root contains more than 60 identified natural chemicals, including stilbenes, such as resveratrol and polydatin, and anthraquinones, such as emodin and physcion. In laboratory and small human studies, these chemicals showed antioxidant and anti-inflammatory effects, and they may help reduce the activity of certain immune cells. However, more research is needed to know if these effects provide meaningful benefits in people.

The benefits of knotweed for most uses are not well defined. Research on blood sugar control in people with type 2 diabetes has produced mixed results. Some studies found that resveratrol lowered fasting blood sugar levels and improved insulin sensitivity, especially at higher doses, while a review of the high quality studies found no effect on blood sugar levels, though the evidence was limited. Evidence is more consistent for a modest lowering effect on blood pressure and cholesterol.

Common knotweed or knotgrass (Polygonum aviculare) is a related plant that is sometimes sold as knotweed. Unlike Japanese knotweed, it contains very little resveratrol. Its main natural chemicals are flavonoids, such as quercetin, myricetin, kaempferol, and avicularin. Because the two plants contain different chemicals, they are used in different ways. Most research on common knotweed comes from laboratory and animal studies rather than studies in people, so how it works in the body is not well understood.

Herbal products such as knotweed naturally contain many different chemicals, such as resveratrol. The amounts of each chemical in a specific knotweed product may vary. Different amounts of resveratrol may change the effects of knotweed on the body. Some knotweed dietary supplements are made to contain a certain amount of these substances. There are also some dietary supplements made from knotweed that only contain resveratrol.

How should I store knotweed?

You should store knotweed 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 Knotweed?

The most common side effects of knotweed are listed below. The side effects listed below come mainly from research on Japanese knotweed and resveratrol; little is known about the side effects of knotgrass. These side effects may vary depending on the dose you are taking or the specific product you are using. Stomach-related side effects are more likely with higher doses. Tell your health care provider if you have any of these side effects that bother you.

  • Stomach pain
  • Nausea or vomiting
  • Diarrhea
  • Upset stomach (or indigestion)
  • Gas
  • Headache

There may be other side effects of knotweed 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 knotweed?

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

Severe Allergic Reactions. Knotweed may cause allergic reactions, which can be serious. Stop taking knotweed 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

Liver Damage. Knotweed naturally contains a substance called emodin. In laboratory and animal research, emodin has caused liver damage (hepatotoxicity), especially at high amounts and with long-term use. It is not clear if this also happens in humans. Call your health care provider right away if you have any of the following symptoms of liver damage.

  • Nausea or vomiting
  • Stomach or belly pain
  • Fever
  • Weakness or unusual tiredness
  • Itching
  • Loss of appetite
  • Light-colored poop
  • Dark-colored urine (pee)
  • Your skin or the whites of your eyes turning yellowish (also called jaundice)

Warnings & Precautions

Who should not use knotweed?

Allergies to Ingredients. People who are allergic to any of the following should not take knotweed.

  • Common knotweed
  • Fallopia japonica
  • Giant knotweed
  • Hu Zhang
  • Japanese knotweed
  • Knotgrass
  • Polygonum aviculare
  • Polygonum cuspidatum
  • Prostrate knotweed
  • Reynoutria japonica
  • Any of the ingredients in the specific product

Your pharmacist can tell you all of the ingredients in the specific knotweed products they stock.

What should I know about knotweed before using it?

Knotweed is a dietary supplement. It cannot be marketed to treat or cure a disease.

Knotweed is available in many forms, including capsules, tablets, powders, and liquid extracts. It is also commonly sold as a purified resveratrol supplement, which contains only that one natural chemical instead of the full mix of chemicals found in the whole root. Read and follow the directions on the specific product you are taking, or take it as recommended by your health care provider.

Keep knotweed out of the reach of children.

Knotweed supplements are usually made from the root (rhizome) of the plant, which naturally contains the highest amounts of resveratrol and other active chemicals. Products made from other parts of the plant may contain different amounts of these chemicals.

Products sold as knotweed may be made from either Japanese knotweed (Polygonum cuspidatum) or common knotweed (knotgrass or Polygonum aviculare). These are different plants with different natural chemicals and uses. Check the product label for the scientific name so you know which plant you are taking.

Herbal products, such as knotweed, naturally contain many different chemicals. The amount of each chemical in a specific knotweed product can change for many reasons, including the area where the plant was grown, the weather during growth, the time of year when it was harvested, and the way that the plant was processed after it was harvested. If you switch to a new bottle or a different brand, you may not have the same effects or side effects because of this variability. Talk to your health care provider if you have questions or concerns.

The FDA has not tested knotweed 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 knotweed?

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 Conditions. Some knotweed products contain resveratrol, which may act like the hormone estrogen in the body. If you have a condition that could be made worse by estrogen, talk to your health care provider before taking knotweed. Examples of hormone-sensitive conditions include breast cancer, uterine cancer, ovarian cancer, endometriosis, and uterine fibroids.

Current and Past Health Conditions. Tell your health care provider if you have or have had any of the following.

  • Liver problems
  • A bleeding disorder
  • Diabetes or trouble with low blood sugar
  • Low blood pressure

Other Medicines and Supplements. Before taking knotweed, 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 knotweed could affect pregnancy or harm a fetus. Tell your health care provider if you are pregnant or plan to become pregnant.

Breastfeeding. It is not known if knotweed passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed.

Interactions

Does knotweed interact with foods or drinks?

There are no known interactions between knotweed and foods or drinks.

It is unknown if drinking alcohol will affect knotweed, but both knotweed and alcohol may lead to liver damage. It is best to limit the amount of alcohol you drink.

Does knotweed 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.

The interactions between knotweed and medicines are not fully understood. In laboratory and animal studies, chemicals in knotweed (especially resveratrol) have been shown to affect the way the body processes certain medicines. This could change how well those medicines work or increase the risk of side effects.

As with most dietary supplements, the research on drug interactions with knotweed is incomplete. Always check with your health care provider before taking knotweed with any medicines.

Overdose/Missed Dose

What should I do if I accidentally use too much knotweed?

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

Read the product label closely and speak with a health care provider about how to use knotweed safely. Do not take double or extra doses, even if you miss a dose.