Grapefruit also interacts with erectile dysfunction drugs. All of them. Such drugs include Viagra (sildenafil), Cialis (tadalafil), and Levitra (vardenafil).
These drugs, known as the phosphodiesterase inhibitors, already feature a warning about possible prolonged erections, a painful, serious, and potentially damaging side effect known as priapism. Grapefruit could make that worse and it could also cause headaches, flushing, and low blood pressure. One whole grapefruit or a 6- to 7-ounce glass of juice is enough for drug toxicity, according to a 2013 study.
“With grapefruit, you increase the chance of having an emergency visit,” says Dering-Anderson. “My advice is, don't take grapefruit.”