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Grapefruit juice enhances the effect of some commonly used medications by increasing their bioavailability or half life via the selective down-regulation of a specific subfamily of the cytochrome P450 enzyme system in the small intestine- most notably CYP3A4. This effect can be seen with a single grapefruit or 200mL of grapefruit juice, and persist for as much as 3 days.

Grapefruit-drug interactions typically cause increased amounts of the drug in the body, however in a few cases it can cause a decrease.

A number of recreational are affected by grapefruit. Notable examples include:

  • Most benzodiazepines- although the effect may be very minor depending on the specific drug.
  • Ketamine when used intranasally or orally, but not when used IM or IV, has increased absorption and duration from grapefruit.
  • Oxycodone- which can have the half life increased by 50%
  • Methadone, but only to a very small extent.
  • Buprenorphine, but the extent of the effect is unknown.
  • Other opioids may also be affected but more research is needed.
  • Erectile dysfunction drugs Viagra, Cialis and Levitra show somewhat increased blood levels.
  • Cocaine serum levels may be increased, but has not been well studied.