INTERACTIONS OF GRAPEFRUIT WITH MEDICATIONS
Interaction of Grapefruit Juice with Antihistamines
The most significant interaction of grapefruit juice occurred with
anithistamines. Do not take grapefruit juice if you are taking terfenadine (seldane) or astemizole (Hismanal) due to the possibility of fatal cardiac arrhythmias. Other non-sedating antihistamines are available which do not appear to interact with grapefruit juice. (e.g., loratidine (Claritin) cetirizine
(Reactine) or fexofenadine (Allegra).
In Australia, it is routine practice to add auxiliary labelling to prescriptions for terfenadine warning of grapefruit-juice drug interactions. In the United States, the manufacturer of terfenadine has changed its prescribing information to indicate that GJ should not be taken with
The most significant documented grapefruit juice drug interaction occurred with terfenadine
(Seldane). A previously healthy 29-year old male used terfenadine twice daily for one year to treat allergic
rhinitis. The patient drank grapefruit juice two to three times weekly. On the day of his death, he consumed two glasses of juice, took his terfenadine dose, and proceeded to mow the lawn. He became ill, collapsed and died. Post-mortem terfenadine and terfenadine metabolite plasma levels were reported as 35
ng/mL and 130 ng/mL respectively.
(In normal cases, the plasma concentration is not detectable.) These levels are within range of previously noted arrhythmogenic levels of
terfenadine. The individual had both cardiomegaly and hepatomegaly, but no evidence of impaired hepatic function.
To study the effect of grapefruit juice on terfenadine 12 healthy subjects were given terfenadine 60 mg with either water or grapefruit juice, co-administered with terfenadine or delayed by 2 hours.
|None of the 12 had quantifiable levels when the drug was given with water. |
|Of subjects who took terfenadine co-administered with grapefruit juice, 100% had quantifiable terfenadine levels.|
|Of subjects who delayed grapefruit juice ingestion by 2 hours, 33% had quantifiable levels. |
|In the group of subjects which co-administered grapefruit juice and
terfenadine, a significant mean prolongation in the QT interval measured by electrocardiogram was demonstrated. |
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