Grapefruit may be dangerous if you are taking an erectile dysfunction drug
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.”
Food – Health.com