It usually takes several days to a week to get the results of cytochrome P450 tests. Once they're back, you and your doctor will discuss the results and how they might affect your treatment options.

Cytochrome P450 tests give clues to how well your body processes a drug by looking at specific enzymes. People can be classified according to how fast they metabolize medications. For example, results of a 2D6 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 can increase the likelihood that it 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 normal metabolizers do. This can increase your risk of side effects and drug interactions.
  • Normal 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.

Cytochrome P450 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 2D6 enzyme is involved in metabolizing antidepressants such as fluoxetine (Prozac), paroxetine (Paxil), venlafaxine (Effexor XR), and tricyclic antidepressants such as nortriptyline (Pamelor), amitriptyline, clomipramine (Anafranil), desipramine (Norpramin) and imipramine (Tofranil).
  • The 2C19 enzyme is involved in metabolizing citalopram (Celexa) and escitalopram (Lexapro).
  • The antidepressant desvenlafaxine (Pristiq) bypasses the 2D6 system, so it's not metabolized by 2D6.

Test limitations

Although they have potential, cytochrome P450 tests have limitations, including those below.

  • Cytochrome P450 testing can't predict for certain which particular medication will work best for you — it can only provide clues.
  • Cytochrome P450 tests only look at some of the genes involved in how your body uses certain drugs — so factors out of the scope of this test may impact how an antidepressant will affect you.
  • Because they're still being developed, it isn't entirely clear how useful cytochrome P450 tests are in choosing an antidepressant.
  • An advisory group formed by the Centers for Disease Control and Prevention has concluded that there isn't enough evidence yet to support using cytochrome P450 testing for the most commonly prescribed antidepressants.
  • Even if you have cytochrome P450 tests, you may still need to try different antidepressants and different doses to identify what works best for you.

Despite the limitations, some doctors use these tests and think they're helpful. But cytochrome P450 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.

April 16, 2015