Except for being high in beta-carotene, pink and red varieties are similar to other types of grapefruit. Along with bioflavonoids, pectin and vitamin C, grapefruit contains furancoumarin, a chemical substance that impacts the timing and absorption of some medications by inhibiting a digestive enzyme. Effects seem to be dose-dependent; a glass of grapefruit juice may produce drug interactions while eating half a grapefruit may not. You should consult a pharmacist or physician for questions about specific medications, grapefruit and grapefruit juice.
Blood Pressure Medications
Nifedipine and amlodipine are calcium channel blockers used to treat high blood pressure. Japanese researchers tested the effects of grapefruit and grapefruit juice on a 50-year-old hypertensive man taking these medications. The subject consumed one grapefruit prior to taking each drug, washing the pill down with 500ml, or about 10.6 oz., of grapefruit juice. Blood pressures were measured over a period of time. No effects on amlodipine levels were detected; however, nifedipine concentrations were rapidly increased in the blood, causing a rapid drop in blood pressure. The effects were short-acting. Consumption of whole grapefruit did not impact the actions of either medication. The study was published in the January 2010 issue of "Clinical and Experimental Hypertension." Grapefruit juice may increase the bioavailability of other calcium channel blockers, including felodipine and diltiazem.
Benzodiazepines, or minor tranquilizers, are classified as hypnotic drugs. In a study published in the March 2006 "European Journal of Clinical Pharmacology," two benzodiazepines, triazolam or quazepam, were given to nine healthy volunteers in four separate trials, with and without grapefruit juice. Blood levels of the drugs and physical effects were monitored. Grapefruit juice increased blood concentrations of both medications, with effects more pronounced for triazolam. Sedative actions of both drugs were similar and did not seem to be enhanced by the grapefruit juice.
Some cholesterol-lowering medications may interact with grapefruit juice. Randomized two-way crossover studies lasting three days each in Tokyo compared atorvastatin and pravastatin taken with either grapefruit juice or water. Subjects were divided into two groups of 10 and given either grapefruit juice or water three times a day for two days, followed by a single 10mg dose of medication and approximately 8 oz. of juice or water before lunch or dinner. Results, published in the April 2004 "British Journal of Clinical Pharmacology," showed elevated blood concentrations of atorvastatin associated with grapefruit juice, while no significant effects were measured when pravastatin was taken with juice.
A synthetic opioid, methadone is used to treat heroin addiction. For patients with chronic pain, careful titration of methadone during the first few days is needed to produce analgesia without overdose. Researchers in Switzerland gave addiction clinic patients water or grapefruit juice before and after methadone doses for five days. Study results, published in the July 2004 "Clinical Pharmacology and Therapeutics," measured a modest increase in peak methadone levels and a decrease in drug clearance. Patients had no symptoms of overmedication. Researchers urged caution as potentially stronger effects could occur during initial therapy for pain management.
- "Clinical and Experimental Hypertension"; Effects of Ingestion of Grapefruit Juice or Grapefruit on the Hypotensive Effect and Plasma Concentrations of Dihydropyridine Calcium Antagonists (Amlodipine and Nifedipine): A Case Study; K. Nakagawa, T, Goto, January 2010
- "European Journal of Clinical Pharmacology"; Interaction Between Grapefruit Juice and Hypnotic Drugs: Comparison of Triazolam and Quazepam; K. Sugimoto et al.; March 2006
- "British Journal of Clinical Pharmacology"; Effects of Grapefruit Juice on Pharmacokinetics of Atorvastatin and Pravastatin in Japanese; I. Fukazawa et al., April 2004
- "Clinical Pharmacology and Therapeutics"; Effects of Grapefruit Juice on the Pharmacokinetics of the Enantiomers of Methadone; M. Benmebarek et al.; July 2004