Spokesman and clinical pharmacologist Dr. Evan Begg said a warning had gone out to the country's doctors and pharmacists, and all affected medicines will now carry warning labels in New Zealand. Two of the most widely used drugs affected are oral contraceptives and some non-sedating antihistamines. Dr. Begg said grapefruit juice is able to boost the concentration of some drugs by up to nine times, with its effect lasting up to 10 hours. One of the most dangerous interactions is with antihistamines such as terfenadine. If someone was taking terfenadine and drinking grapefruit juice every morning, the risk of cardiac problems would be greatly increased, said Dr. Begg.
Unwanted pregnancies are another possible consequence.
A woman on the Pill who drank or ate grapefruit every breakfast would have increased hormone levels thanks to the fruit, and if she stopped drinking or eating it, her hormone levels would drop. Dr. Begg said the effects were found only with grapefruit juice, and not orange or other citrus juices. International research has shown for some time the effects of grapefruit juice, and it was time a concerted effort was made to alert the public, doctors and pharmacists, he said.