Your doctor may use cytochrome P450 (CYP450) tests to help determine how your body processes (metabolizes) a drug. The human body uses cytochrome P450 enzymes to process medications. Because of inherited (genetic) traits that cause variations in these enzymes, medications may affect each person differently.

Drug-gene testing — also called pharmacogenomics or pharmacogenetics — is the study of how genes affect your body's response to medication. Tests look for changes or variations in these genes that determine whether a medication could be an effective treatment for you or whether you could have side effects from a specific medication.

The cytochrome P450 enzyme includes the CYP2D6 enzyme, which processes many antidepressants and antipsychotic medications. By checking your DNA for certain gene variations, CYP450 tests can offer clues about how your body may respond to a particular antidepressant. CYP450 tests can also identify variations in other enzymes, such as the CYP2C19 enzyme.

CYP450 and other genetic tests (genotyping tests) are becoming more common as doctors try to understand why antidepressants help some people and not others. While the use of these tests might be increasing, there are limitations.

Medications for depression are usually prescribed based on symptoms and medical history. For some people, the first antidepressant tried relieves depression symptoms and has tolerable side effects. For many others, however, finding the right medication takes trial and error. For some people, it can take several months or longer to find the right antidepressant.

Genotyping tests, such as cytochrome P450 tests, may speed up the identification of medications that are more likely to be better processed by your body. Ideally, better processing would lead to fewer side effects and improved effectiveness. CYP450 tests are generally used only when initial antidepressant treatments aren't successful.

Genotyping tests are also used in other areas of medicine. For example, the CYP2D6 test can help determine whether certain cancer medications, such as tamoxifen for breast cancer, are likely to be more effective. The CYP2C9 test can help determine appropriate dosing of the blood thinner warfarin to reduce the risks of adverse effects.

The field of pharmacogenomics is growing and many different types of genotyping tests are available. Tests differ widely by which classes of drugs they examine and how the tests are performed.

You won't need to fast or have any special preparations before the procedure.

For cytochrome P450 tests, a sample of your DNA is taken, using one of these methods:

• Cheek swab. A cotton swab is rubbed inside your cheek to get a cell sample.
• Saliva collection. You spit saliva into a collection tube.
• Blood test. Blood is drawn from a vein in your arm.

After the procedure

Getting the DNA sample should take just a few minutes. Then your sample is sent to a lab where your DNA is analyzed for specific genes.

It usually takes several days to a week to get the results of cytochrome P450 tests. You and your doctor can discuss the results and how they might affect your treatment options.

CYP450 tests give clues about how well your body processes a drug by looking at specific enzymes. The results can be classified according to how fast you metabolize a specific medication. For example, results of a CYP2D6 test may show which of these four types applies to you:

• Poor metabolizers. If you process a certain drug more slowly than normal because of a missing enzyme, the medication can build up in your system. This buildup can increase the likelihood that the medication will cause side effects. You might still be able to benefit from these medications, but at lower dosages.
• Intermediate metabolizers. If you have reduced enzyme function in processing drugs, you may not process some medications as well as do people who are classified as normal metabolizers.
• Normal metabolizers (also known as extensive metabolizers). If the test shows that you process certain antidepressants normally, you're more likely to benefit from treatment and have fewer side effects than people who don't process those particular medications as well.
• Ultrarapid metabolizers. In this case, medications leave your body too quickly — often before they have a chance to work properly. You'll likely need higher than usual doses of medications.

CYP450 testing isn't useful for all antidepressants, but it can provide information about how you're likely to process a number of them. For example:

• The CYP2D6 enzyme is involved in metabolizing antidepressants such as fluoxetine (Prozac), paroxetine (Paxil, Pexeva), fluvoxamine (Luvox) and venlafaxine (Effexor XR), as well as tricyclic antidepressants such as nortriptyline (Pamelor), amitriptyline, clomipramine (Anafranil), desipramine (Norpramin) and imipramine (Tofranil). Some antidepressants, such as fluoxetine and paroxetine, can cause the CYP2D6 enzyme to slow down.
• The CYP2C19 enzyme is involved in metabolizing citalopram (Celexa) and escitalopram (Lexapro).

Test limitations

Although they have potential, CYP450 tests have limitations:

• Tests are available for only certain medications. Also, each test is specific to only one medication, so a change in medication may require another test.
• Testing can't predict which medication will work best for you — it can only provide clues.
• Tests look at only some of the genes involved in how your body uses certain drugs — so factors out of the scope of the test may impact how an antidepressant will affect you.
• Tests focus on how the body metabolizes a drug, not on what the drug does to the body, such as how the drug might change the brain receptors to improve symptoms.
• It isn't entirely clear how useful tests are in choosing an antidepressant. Even if you have tests, you may still need to try different antidepressants and different doses to identify what works best for you.
• Not all insurance companies cover the costs of these tests.

Despite the limitations, some doctors use CYP450 tests and think they're helpful. However, these tests aren't meant to be the only way to determine which antidepressants to try. They're just one tool that may help. Trying antidepressants based on your medical history and symptoms is still the standard method for identifying the best medication for your needs.