What is Cashew bark?

Cashew bark is the bark of the cashew tree, Anacardium occidentale. In Yoruba the tree is called kaju. Healers boiled the bark in water to make a bitter brown wash or drink.

It is not the nut. The nut is a food, sold everywhere. The bark is a folk remedy, used for loose stools, sore mouths, toothache and "sugar sickness".

Here is the honest verdict. Human evidence for cashew bark is very thin. Almost everything known comes from test tubes and animals, and long use in tradition is not proof that it works or that it is safe.

How does Cashew bark work?

No one has shown how cashew bark works in people. The ideas below come from lab and animal work only.

The bark is rich in tannins. Tannins are astringent, meaning they bind proteins and pucker tissue. In lab work this can firm up gut lining and slow fluid loss, which may be why the old use was for diarrhoea.

In test tubes, bark and leaf extracts slow the growth of some bacteria and yeasts. Whether that happens inside a person is not established.

In rats, stem bark extracts have lowered blood sugar. The doses and the animals are not the same as a cup of bark tea. No trial has tested this in people with diabetes.

The bark also holds anacardic acids. In cell studies these block certain enzymes. The same compounds can burn skin, so a lab effect here is not good news on its own.

What forms does Cashew bark come in?

Cashew bark is sold mostly as dried bark chips or a coarse powder. Tinctures and capsules exist but are uncommon.

In West Africa the usual form is a boiled bark water. The same brew was also used as a mouth rinse or a wash for skin.

Never confuse bark products with cashew nut shell liquid. That is a caustic oil that blisters skin. Check the label for the plant part before you buy anything.

Bark products are not tested the way medicines are. No regulator checks their strength or purity.

Where it comes from:

Cashew trees came from Brazil. Portuguese traders carried them to West Africa in the 1500s. The tree spread fast and became a common village and roadside plant.

Yoruba and other West African healers used bark and leaf brews for loose stools, mouth sores, toothache and high blood sugar. Washes were also used on wounds.

Similar uses were written down in Brazil, India and the Caribbean. The knowledge passed by word of mouth and later into regional plant handbooks.

What does Cashew bark help with?

• Diarrhoea (loose stools): This was the most common folk use. The tannins in the bark are astringent, which in lab work firms tissue and slows fluid loss. No human trial has tested cashew bark for diarrhoea, so the benefit is unproven.
• Mouth and gum sores: Healers used cooled bark water as a rinse for sore gums and mouth ulcers. Lab tests show bark extracts slow some mouth bacteria. There is no good human evidence that rinsing speeds healing.
• Toothache: Bark was chewed or held against a painful tooth. Any relief may come from the bitter, drying taste rather than true pain relief. Toothache needs a dentist, because the cause does not go away on its own.
• High blood sugar in type 2 diabetes: Folk use for "sugar sickness" is widespread across West Africa. Rat studies show lower blood sugar after stem bark extracts. There are no human trials, and cashew bark must not replace diabetes medicine.
• Wound and skin washes: Bark brews were used to wash cuts and sores. In test tubes the extracts slow some bacteria. On broken skin the irritant compounds in the bark may do harm, and this use is not supported by human studies.
• Fever in malaria: Bark and leaf brews were given for fever in some regions. Lab work on related extracts against malaria parasites is early and mixed. Malaria is a medical emergency and needs tested drugs, not a bark tea.

What are the side effects of Cashew bark?

• Nausea and stomach upset
• Constipation from the tannins
• Dry, puckered feeling in the mouth
• Reduced iron absorption with regular use
• Rash, itching or blisters after handling the raw bark
• Allergic reaction in people allergic to cashew, pistachio or mango
• Low blood sugar if you also take diabetes medicine

Good to know:

Mild stomach upset is the most likely problem. It usually settles once you stop.

The serious risks are allergy and skin burns. People allergic to cashews can react to the tree itself. The irritant oils are strongest in the nut shell, but the bark holds them too.

Side effects have never been tracked in a proper study. So rare harms would not yet be known.

How much Cashew bark should you take?

• Recommended daily: There is no established amount for cashew bark, and most products carry no meaningful dose at all. Strength changes with the tree, the season and the batch. Where a label does give a figure, it is usually a small spoonful of dried bark simmered in water, once or twice a day. That number comes from custom, not from testing in people. It is a note about what labels say, not a plan for you to follow.
• Best time of day: Time of day does not matter much here. The bark is bitter and drying, so many people take it with a meal. Keep it well away from iron tablets. Tannins bind iron in the gut and can cut how much you absorb.
• With or without food: Food is the safer choice. Taking bitter, tannin-rich brews on an empty stomach often causes nausea. If you take iron, or a thyroid or antibiotic tablet, keep a gap of at least two hours. Tannins can bind these drugs in the gut.
• Time to effect: No reliable answer exists, because the effects have not been measured in people. An astringent brew for loose stools would be expected to act within hours, if it acts at all. Any blood sugar effect is unproven in humans. It is fair to expect that cashew bark may do nothing for you.
• How long to take it: An honest answer is that no safe length of use has been worked out. In tradition it was used for a few days, for a short problem, then stopped. Long daily use is the bigger worry. Heavy tannin intake can block iron and upset the stomach over weeks. Nobody has studied what months of cashew bark does to the liver or kidneys.