What are P450 inhibitors?

P450 refers to a family of metabolic proteins called cytochrome P450 enzymes; they are responsible for breaking down drugs within the body. P450 inhibitors slow or stop the chemical action of a cytochrome P450 enzyme, usually by binding to the enzyme before it can do its job. Certain medications are known to be P450 inhibitors, but some foods can also inhibit these enzymes. It is helpful in some cases to purposely inhibit P450 enzymes because they break down some drugs before the drug can work. In other cases, drugs and foods that inhibit enzyme function must be avoided so that a drug breaks down quickly enough and does not accumulate in the body.

Large concentrations of cytochrome P450 enzymes are found in the liver, but they can be found in many other organs such as the kidneys and digestive tract. These enzymes start the process of breaking down many different substances that can become toxic if not excreted in urine or bile. Substances include medications, illegal drugs, steroid hormones and other organic molecules.

A wide range of medications act as P450 inhibitors. This inhibition, along with the opposite effect, induction, is responsible for most drug interaction problems and can cause some drugs to reach toxic levels in the body. Many antidepressants like Prozac, Zoloft and Luvox are inhibitors of certain P450 enzymes, along with Taget, Cipro, Benadryl and a long list of other drugs. Consumption of grapefruit, grapefruit juice, star fruit, watercress and even the use of tobacco products also inhibit these enzymes.

Some drugs are no longer used in the United States due to potentially fatal side effects caused by interactions with other P450 inhibitor drugs. Examples include Seldane, Hismanal, Propulsid. Another drug, Posicor, was discontinued because it is a strong P450 inhibitor that reacted dangerously with some heart medications.

An important point is that some drugs are less effective because they metabolize faster, while other drugs are actually more effective. Drugs that are already in chemical form and are biologically active do not relieve symptoms, even though there is high cytochrome P450 activity. They may work better if taken with P450 inhibitors. Other drugs are taken inactively and are activated in the early stages of decomposition. These drugs may not work as well if the patient is also taking a cytochrome P450 inhibitor.

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