Medicines for depression, called antidepressants, usually are prescribed based on symptoms and medical history. For some people, the first antidepressant that's tried eases depression symptoms, and side effects don't cause major problems. For many others, finding the right medicine takes trial and error. Sometimes it can take several months or longer to find the right antidepressant.

CYP450 tests can identify variations in many enzymes, such as the CYP2D6 and CYP2C19 enzymes. The CYP2D6 enzyme processes many antidepressants and antipsychotic medicines. Other enzymes such as the CYP2C19 enzyme also process some antidepressants. By checking your DNA for certain gene variations, CYP450 tests that include CYP2D6 tests and CYP2C19 tests can offer clues about how your body may respond to a specific antidepressant.

Genotyping tests, such as cytochrome P450 tests, may speed up the time it takes to find medicines that the body can process better. Ideally, better processing leads to fewer side effects and works better to ease symptoms. CYP450 tests for depression are generally used only when the first antidepressant treatments aren't successful.

Genotyping tests also are used in other areas of medicine. For example, the CYP2D6 test can help find out if certain cancer medicines, such as tamoxifen for breast cancer, are likely to work well. Another CYP450 test, the CYP2C9 test, can help find the best dose of the blood thinner warfarin to lessen the risks of side effects. But your healthcare professional may suggest another type of blood thinner.

The field of pharmacogenomics is growing, and many genotyping tests are available. CYP450 tests are becoming more common as healthcare professionals try to understand why antidepressants help some people and not others. Tests differ widely by which types of medicines they look at and how the tests are done. While the use of these tests might be increasing, there are limitations.

You can buy at-home pharmacogenetic test kits. These direct-to-consumer tests are available without a prescription. The tests vary greatly in what genes they look at and how the results are given. The accuracy of these at-home tests is not always clear, and they're not usually helpful in deciding on medicine options. If you choose to use an at-home test kit, it's best to bring the results to a healthcare professional or pharmacist familiar with this type of testing. Together you can talk about the results and what they mean for you.

Cheek swab, saliva and blood tests have almost no risk. The main risk with blood tests is soreness or bruising at the site of the blood draw. Most people don't have serious reactions to having blood drawn.

Before the cheek swab test, you may be asked to wait 30 minutes after eating, drinking, smoking or chewing gum.

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. A sample of blood is taken 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 looked at for specific gene variations.

It usually takes several days to a week to get the results of cytochrome P450 tests. You can talk with your healthcare professional or pharmacist about the results and how they might affect your treatment options.

CYP450 tests give clues about how well your body uses and gets rid of medicine by looking at specific enzymes. How the body uses and gets rid of medicine is called processing or metabolizing. The results can be grouped according to how fast you metabolize a specific medicine. For example, results of a CYP2D6 test may show which of these four types applies to you:

• Poor metabolizer. If you're missing an enzyme or have too little of it, you may process a certain medicine more slowly than other people do. The medicine can build up in your system. This buildup can raise the likelihood that the medicine will cause side effects. You may benefit from this medicine, but at lower doses.
• Intermediate metabolizer. If the test shows that the enzyme doesn't work quite as well as intended, you may not process some medicines as well as people who are called extensive metabolizers. But how well a medicine works for intermediate metabolizers is typically much like that for extensive metabolizers.
• Extensive metabolizer. If the test shows that you process certain medicines as intended and in the most common way, you're more likely to benefit from treatment and have fewer side effects than people who don't process those specific medicines as well.
• Ultrarapid metabolizer. In this case, medicines leave your body too quickly, often before they have a chance to work as they should. You'll likely need higher than usual doses of these medicines.

CYP450 tests also can give information about medicines that need to be processed to their active forms by the cytochrome P450 enzyme so that they can work. These medicines are called prodrugs. For example, tamoxifen is a prodrug. It must be metabolized or activated before it will have the desired effect. A person who doesn't have enough working enzyme and is a poor metabolizer may not be able to activate enough of the medicine for it to work as it should. A person who is an ultrarapid metabolizer might activate too much of the medicine, possibly causing an overdose.

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

• The CYP2D6 enzyme is involved in processing antidepressants such as fluoxetine (Prozac), paroxetine (Paxil), fluvoxamine (Luvox), venlafaxine (Effexor XR), duloxetine (Cymbalta, Drizalma Sprinkle) and vortioxetine (Trintellix). The enzyme also is involved in processing tricyclic antidepressants such as nortriptyline (Pamelor), amitriptyline, clomipramine (Anafranil), desipramine (Norpramin) and imipramine. Some antidepressants, such as fluoxetine and paroxetine, also can cause the CYP2D6 enzyme to slow down.
• The CYP2C19 enzyme is involved in processing citalopram (Celexa), escitalopram (Lexapro) and sertraline (Zoloft).

Test limitations

Although they have potential, CYP450 tests have limitations:

• Tests are available only for certain medicines. Also, each test is specific to only one medicine, so a change in medicine may need another test.
• Testing might give an idea of whether you may have a lower, standard or higher level of medicine in your body, but it can't tell for sure which medicine will work best for you. It can only give clues.
• Tests look at only some of the genes involved in how your body uses certain medicines. Other factors that are not part of the test may play a role in how an antidepressant may affect you.
• Tests focus on how the body processes a medicine, not on what the medicine does to the body. For example, the test doesn't give information on how the medicine might change brain receptors to help ease symptoms.
• It's not 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 find out what works best for you.
• Not all insurance companies cover the costs of these tests.

Despite the limitations, some healthcare professionals use CYP450 tests and think they're helpful. But these tests aren't meant to be the only way to decide 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 finding the best medicine to meet your needs.