About cytochrome P450 genotyping test

The cytochrome P450 genotyping test (CYP450 test) predicts how your body will process (metabolize) certain antidepressants. The CYP450 test is one of the newer entries in the emerging field of pharmacogenomics, or personalized medicine.

Before the CYP450 test, doctors had little way of knowing how you would respond to a particular antidepressant — whether you would develop troubling side effects, for instance, or whether the medication would even be of benefit. By analyzing how specific genes in your liver break down medications, the CYP450 test may remove some of the guesswork.

The CYP450 test isn’t yet widely available. But as genotyping tests such as CYP450 become more well-known, they may offer better options for the treatment of depression.

As with other medications, your doctor usually chooses an antidepressant based on your medical history and symptoms. Though these are solid guidelines, they can’t predict how you will react to the medication or whether it will be effective for you. As a result, you may spend months or even years trying different drugs before you find the right treatment. It’s hoped that genotyping tests such as the CYP450 will help circumvent some of this frustrating trial-and-error process.

How the test works
The CYP450 test involves a series of specific genotyping tests to evaluate genes (CYP2D6, CYP2C19 and CYP2C9, for example.) that produce enzymes your body needs to process certain antidepressants and other medications. The test can’t predict how effective a particular medication will be for you. Instead, it shows which drugs are more likely to be effective, based on your metabolism. Just as important, it can minimize the risk of serious side effects.

According to some studies, antidepressants cause severe reactions in about 8 percent of people who take them and moderately severe reactions in another 30 percent. Knowing in advance how your body is likely to metabolize an antidepressant means that your doctor has a better chance of prescribing the right medication and dosage.

What you can expect

During the test
The CYP450 is a simple blood test. Blood is drawn from a vein in your arm and sent to a laboratory for testing. You may feel a slight prick when the needle is inserted. You won’t need to fast or undergo any special preparations prior to the test. You don’t even have to change into a gown.

After the test
After the blood test, which should take just a few minutes, you can resume your normal activities. The blood draw itself carries virtually no risk, though you may have slight swelling or redness at the puncture site.


It usually takes about two to five days to get the results of the CYP450 test. You and your doctor will discuss the results and how they might affect your treatment options.

The CYP450 test divides people into four main types:

  • Normal metabolizers. If the test shows that you process certain antidepressants normally, you’re more likely to benefit from treatment and to have fewer side effects than are people who don’t metabolize medications as well.
  • Intermediate metabolizers. Because you have at least one gene involved in drug metabolism that doesn’t function normally, you may not process some medications as well as normal metabolizers do.
  • Slow metabolizers. If you metabolize certain drugs more slowly than normal, medications can build up in your system, with the potential for serious side effects.
  • Ultrarapid metabolizers. In this case, medications leave your body too quickly — often before they have a chance to work properly.

Test limitations
Although they may be a good first step in personalized medicine, genotyping tests have limitations. One is that they can’t predict if certain medications will actually work — they can only show which probably won’t work. In addition, the CYP450 test only looks at some of the genes involved in metabolizing drugs. Not all antidepressants are processed by these genes. Those that are included:

  • Amitriptyline
  • Clomipramine (Anafranil)
  • Desipramine (Norpramin)
  • Imipramine (Tofranil)
  • Paroxetine (Paxil, Paxil CR)

Finally, genotyping tests aren’t yet available in all communities nor do all doctors use them. And some health insurance plans don’t cover the tests, so check with your health insurance provider. If you opt for testing, keep in mind that the CYP450 genotyping test isn’t meant to be the only way to determine which antidepressants to try. A thorough medical and psychiatric evaluation, as well as consideration of your preferences and lifestyle, is equally important.