Giant Hogweed and Grapefruit: Health Effects of Furanocoumarins

The giant hogweed can inflict painful burns and blisters on someone who touches it. The sap contains a phototoxic chemical that becomes active when it’s exposed to light. The activated chemical damages the skin. Grapefruit juice is known to interfere with the action of many medicines. It increases their absorption through the lining of the small intestine, which can result in an overdose. The plant chemicals responsible for both the hogweed and the grapefruit juice effect are known as furanocoumarins.

Giant hogweed can be seen in summer where I live and is definitely a plant to watch out for. In fact, we are asked to report it to the local authorities if we see it on public land. Since grapefruit is available all year, it’s always a concern for people who take specific medicines.

Furanocoumarins are toxic chemicals that help to protect plants from pests. A furanocoumarin molecule consists of a furan ring joined to a substance called coumarin. Furan contains a ring made of four carbon atoms and one oxygen atom. Coumarin consists of two six-membered rings. It has a pleasant scent and is used in some perfumes and fabric conditioners. Different furanocoumarin molecules have slight differences in their structure.

Phototoxic Furanocoumarins

Some furanocoumarins are phototoxic, which means that they’re harmful when they contact human skin and are exposed to light. This effect may be familiar to people who handle lime juice, which like the giant hogweed contains furanocoumarins that can cause burns after light exposure. The burns are technically an example of phytophotodermatitis. Phyto is a prefix that means plant. Photo indicates that a process involves light. Dermatitis is inflammation of the skin.

Furanocoumarins and Medications

Bergamottin is a furanocoumarin in grapefruit juice and is responsible for the fruit’s interference with medicines. (A related chemical in grapefruit is thought to play a role in this effect as well.) Bergamottin is found in other citrus fruits besides grapefruit, including bergamot oranges, Seville oranges, and tangelos. A tangelo is a hybrid created from a tangerine and a grapefruit.

The scientific name of the giant hogweed is Heracleum mantegazzianum. It belongs to the family Apiaceae, which was once known as the Umbelliferae. The family also includes celery, carrot, parsnip, parsley, and their wild relatives. The members of the family bear small flowers in clusters known as umbels. The umbels are positioned at the end of stems of approximately equal length that come from a common point. The stems look like the spokes of an upturned umbrella. The surface of a group of umbels is either flat or dome shaped.

Some of the wild members of the family Apiaceae are edible. Collecting these plants for food is a risky endeavour, however, due to the problem of identifying plants that look quite similar. In addition to the giant hogweed, the family contains some extremely poisonous species, including the water hemlock and the poison hemlock. A mistake in identification could be deadly.

A full grown giant hogweed is a very impressive plant that reaches a height of around 6 to 18 feet. It often towers over a human being, as shown in the video below. The plant is frequently likened to the triffids. These giant plants were a major component of “The Day of the Triffids”, a 1951 novel written by John Wyndham. Triffids were menacing creatures that injured or more often killed people with their sting. The triffids could move from place to place and didn’t look like the hogweed, however.

The giant hogweed is native to Eastern Europe and Asia but has been introduced to many other areas, including British Columbia, where I live. Not all of the introductions have been accidental. The plant has been brought into some areas as an ornamental plant. It grows beside roads, streams, and ditches and on waste and agricultural land.

A giant hogweed is often easy to identify because of its height. It grows vigorously in the early stages of its life. Other identification clues are important, though, especially for a shorter or an immature plant. Some of these clues are listed below. The plant has:

  • small white flowers in a flattened or slightly domed cluster
  • up to fifty or more rays per flower cluster
  • a flower cluster that is as wide as two and a half feet
  • green stems that often have purple spots
  • ridges and coarse hairs on the stems
  • hollow stems (although this may not be obvious without breaking the stem and being exposed to the sap)
  • large leaves consisting of three deeply lobed leaflets
  • leaves up to five inches wide

It’s important to remember that some plants in a species may have an atypical appearance. If you have any doubt about the identity of a plant, admire it from a distance and don’t touch it.

A brush with the sap of a giant hogweed can be a very painful experience. Sap (liquid) from any part of the plant is harmful. The redness doesn’t appear immediately. Once the discolouration does appear, it changes in appearance over time. Blisters may appear at some point and in some cases long-lasting scars may be produced.

The sap is dangerous if it gets in the eyes. It can cause severe eye irritation and even blindness. Reports vary in respect to whether the blindness is temporary or potentially permanent, but in either case it’s a serious concern.

Giant hogweed can be especially hazardous for children or animals who are unaware that the plant is dangerous. Some reports say that children like to use bits of hollow stem as a pea shooter or a pretend telescope, which is a scary thought. If the stem releases any sap, photodermatitis will result.

If someone is in a position where they must remove a giant hogweed plant, precautions are necessary. Protective clothing, gloves, and eye protection are essential. A full face mask would be beneficial as well.

If a person does come into contact with hogweed sap, first aid procedures and aftercare are needed.

  • Avoid touching the eyes.
  • Immediately and thoroughly wash the area with soap and water. (This is the standard recommendation. Soap and water may not be available, however, so the area should be cleaned in whatever way is possible. The affected person should then head straight for the nearest source of soap and water.)
  • Cover the area with layers of clothing or a material that blocks sunlight.
  • Wear sunscreen or sun protection clothing over the area for at least several months after exposure. Some reports say that the area may stay especially sensitive to sunlight for several years.
  • Get medical attention if the exposure or effect is widespread or severe. Get medical aid immediately if sap has entered the eyes.

Grapefruit juice often increases the amount of medication that’s absorbed from the small intestine. This might sound like a good idea, since the medication is designed to treat our health problem, but it’s actually bad news. Medicines are helpful chemicals at what is known as the “therapeutic dose” but are often dangerous at high doses. When pharmaceutical companies recommend a dosage for their product, they are taking into account the usual amount that passes through the lining of the small intestine into the bloodstream. Extra medication entering the blood can produce an overdose that results in serious side effects.

CYP3A4 is an enzyme in the small intestine that helps to break down drugs. Grapefruit furanocoumarins inhibit this enzyme, which means that more of the drug can be absorbed through the lining of the small intestine. The effect of the furanocoumarins may last for several hours and may occasionally last for as long as three days. Drinking grapefruit juice either before or after taking the drug can produce the effect. Just one glass of juice or two segments of grapefruit are enough to cause problems.

It used to to be thought that grapefruit juice always acted by increasing the absorption of medications. It’s now known that in some cases it decreases medication absorption. For example, it decreases the absorption of fexofenadine. A common brand name of this medication is Allegra. The drug is an antihistamine that relieves the symptoms of seasonal allergies.

Proteins known as transporters carry drugs into the cells lining the small intestine, enabling the drugs to be absorbed. Grapefruit inhibits some of these transporters, reducing the absorption of the medication. The inhibition is caused by a grapefruit chemical called naringin and not by furanocoumarins, however.

The main classes of medications that are affected by grapefruit juice are listed below. Only some members of each class are known to be affected by grapefruit.

  • statins (a group of drugs prescribed to treat high cholesterol)
  • drugs that fight infections
  • heart medications
  • drugs that lower blood pressure
  • drugs that combat anxiety
  • medications that prevent rejection of transplanted organs
  • medications that fight cancer

At the current time, more than fifty medicines are thought to be influenced by grapefruit. These medicines can be identified by reading the information that comes with a prescribed product. A doctor may be able to substitute a safe medication for one that’s affected by grapefruit juice. Sometimes this isn’t possible or isn’t in the patient’s best interests, however.

There are still many questions about furanocoumarins that need to be answered. How do the phototoxic ones create such a major and prolonged effect on skin biology? (Some bind to the DNA inside cells, but more research is needed.) Which versions of the chemicals are present in a particular plant species? How do their effects on our body differ? Can we create grapefruit plants without the chemicals by selective breeding or hybridization?

Although we need to learn more about furanocoumarins, we already know that the ones in giant hogweed need to be avoided. For people taking certain medications, grapefruit ones need to be avoided as well. In the future, however, some furanocoumarins may prove to be helpful, as psoralen is today. They are interesting chemicals.

  • Giant hogweed facts from the Invasive Species Council of British Columbia
  • Dangers of giant hogweed from the New York State Department of Environmental Conservation
  • Dealing with hogweed burns from the government of King County, Washington
  • Definition of psoralen from the National Cancer Institute
  • Grapefruit-medication interactions from the Canadian Medical Association Journal
  • Grapefruit juice and medicines from the FDA (Food and Drug Administration)