Its official, grapefruit juice is potentially dangerous for subjects with long QT syndrome!
In a study from Israel, subjects were asked to drink 2L of grapefruit juice a day (not everyone can do that!…), and when they did, they put out their QT interval by an average or more than 20ms. If a new medication did this it would be unlikely to get onto the pharmacist’s shelf. There were no adverse events, but this was only over a short period of time. The whole concept of food and drink which causes rhythm problems was discussed in the article, alcohol binging being the commonest one we encounter.
For the future the authors advise doctors to take a history of food and drink taken in before a cardiac arrest, as well as drugs and medications, so we can learn more about this and hopefully prevent some of these events.
For now if you have long QT or carry an at risk gene for it, stick to small amount of grapefruit juice, and if you are someone on another medication which prolongs the QT interval (there are many – see crediblemeds.org), then don’t take grapefruit juice at all.