He is on levitra, (vardenafil) what is exactly the interaction if he drinks grapefruit and what happens to the medicine if is exposed to heat or direct light?..

Toxic: The ingestion of grapefruit/grapefruit juice interferes with an enzyme needed to metabolize vardenafil/Levitra. If this enzyme is interfered with, then too much of the drug may build up in the blood stream & reach an unsafe level. Toxic effects of Levitra include abnormal vision and back pain. Conversely, I would presume that if exposed to heat or direct light the medication might be deactivated.