Yes, it is true that you cannot eat grapefruit or drink grapefruit juice when taking Cialis. This is because grapefruit contains certain enzymes that interact with an enzyme called CYP 3A4. What this enzyme does is break down medications, and the enzymes in grapefruit disable it to the point that the medication isn’t broken down and the man can experience skin redness, cold-like symptoms, headaches, memory problems, or muscle pain.