Some drugs pass through the liver and are transformed by chemicals. This process makes the drugs less toxic and is called metabolism . Many drug interactions have to do with the blockage of or increased rate of metabolism, the blockage of excretion, or a change in absorption. Some drug interactions involve drugs working together or against one another without changing their absorption. These pharmacodynamic interactions can affect how the drug works or its toxicity.
Cytochromes are enzymes that work to change the chemical structure of a drug.
P450 just describes this certain type of cytochromes. A substrate is any drug that is metabolized, or changed, in the liver by one or more of the P450 enzymes. Most HIV medications are P450 substrates that are changed by a specific enzyme, the 3A4 enzyme . This enzyme can also be referred to as the cytochrome P450 3A4 enzyme . Most of the time, once the P450 inhibitor is gone, metabolism goes back to normal.