text: Direct toxicity Many furanocoumarin compounds are toxic. The phytochemicals enter the nucleus of epithelial cells and form a bond (crosslinking) with the DNA when exposed to UV, which causes cell death and causes inflammation via activation of the arachidonic acid cascade. The result is known as phytophotodermatitis, a serious skin inflammation.
  Furanocoumarins produced by plants may serve as a defense mechanism against predators such as insects and mammals. It is also likely that furanocoumarins are related to a plant's natural defense against fungal attack. In particular, the linear furanocoumarins (psoralen, bergapten, and methoxsalen), which occur naturally in Apiaceae, Rutaceae, and other plant families, are known to be toxic to fungi. Plants that cause phytophotodermatitis usually contain linear furanocoumarins.
  Furanocoumarins are found in the sap of plants such as Ammi majus, parsnip, and giant hogweed. At least 36 species of the genus Heracleum in the family Apiaceae are known to contain one or more furanocoumarin compounds.
  Medication interactions
  Furanocoumarins have other biological effects as well. For example, in humans, bergamottin and 6',7'-dihydroxybergamottin are responsible for the "grapefruit juice effect", in which these furanocoumarins affect certain P450 liver and gut enzymes, such as the inhibition of CYP3A4 which either activates or deactivates many drugs, thus leading to higher or lower levels in the bloodstream. Furanocoumarins have various effects which can specifically increase or decrease (depending on the drug) the blood levels of many pharmaceuticals in ways that can be life-threatening, so FDA-approved drugs include warnings for grapefruit.