Cytochrome P450, usually shortened to CYP, is a family of enzymes concentrated in the liver that chemically modify medicines so the body can eliminate them. Individual enzymes are named with a number-letter-number code such as CYP3A4 or CYP2D6.
Many drug interactions happen because one medicine speeds up or slows down a CYP enzyme that another medicine depends on. Grapefruit juice is a familiar dietary example. A pharmacist can screen a complete medication list for these interactions in seconds.