Antidepressants: Test shows which may be your best bet

Figuring out which antidepressant may be best for you has been a matter of trial and error to some extent. To be sure, doctors could make a pretty solid choice based on your medical history, your symptoms and even how your relatives may have responded to the same antidepressant.

But they couldn't predict how the antidepressant would affect you — if you'd wind up with nausea, insomnia or some other side effect, for instance. And you might take a certain antidepressant for several weeks, only to realize your symptoms haven't improved. Doctors also couldn't predict that. So you may have been stuck trying out several different medications over a period of months or even years to find one with the fewest side effects and biggest benefits.

Now the process of choosing an antidepressant may be easier. A relatively new genetic test may help end the sometimes frustrating process of trial and error. This test, called the cytochrome P450 (CYP450) genotyping test, may help you find out how an antidepressant will affect you before you ever swallow the pill.

Testing how your body reacts to antidepressants

The CYP450 test is among a handful of tests that can analyze specific genes that play a role in how your body metabolizes certain medications. It's part of an emerging field called pharmacogenomics, or personalized medicine.

Each of your body's cells contains genes. These genes instruct your body to produce enzymes that control how cells process (metabolize) the medications you take. The CYP450 test identifies the genes responsible for producing enzymes that play an important role in processing certain antidepressants and other medications.

Not everyone processes the same medications the same way. This processing difference is what makes you react differently to an antidepressant than does someone else.

Processing antidepressants too slowly
Your genes may produce enzymes that metabolize an antidepressant too slowly. This means that the medication stays in your body longer than it should. As a result, the medication can build up in your body, possibly leading to intolerable side effects or even a toxic reaction. This, of course, may prompt you to stop taking the medication.

Processing antidepressants too quickly
Your genes may produce enzymes that metabolize an antidepressant too quickly. In this case, the medication is eliminated from your body before it has a chance to work fully. This means that you may not see much improvement in your depression symptoms.

Achieving a balance with antidepressants
If your doctor knows in advance how your body is likely to process an antidepressant, he or she can make a better selection for you or adjust your dose appropriately — before you ever start taking the antidepressant. Your medication choice is personalized to your body's genetics. For instance, if you process an antidepressant too slowly, your doctor may suggest taking a lower dose so that it doesn't accumulate in your body and cause severe side effects.

CYP450 test helps predict reactions to antidepressants

This is where the CYP450 test may help. The test checks to see if two specific genes produce normal enzymes or variants of these enzymes. Which ones you have determine how your body processes certain antidepressants.

Before prescribing an antidepressant, your doctor may discuss with you the option of having the CYP450 test. If you have the test, you and your doctor may know what to expect before you take certain antidepressants. The test helps predict if you're likely to experience harsh side effects or if the antidepressant has little chance of working for you.

The CYP450 test requires only a simple blood draw from a vein in your arm. You don't need to fast before the test or follow other special preparation procedures. You and your doctor will review the results of the test and see how they factor into your choice of antidepressant medication.

Here's how genetic variations can affect processing of an antidepressant and how you and your doctor can decide how to handle them:

Drawbacks of genetic testing for antidepressants

Although it may be a good start in better personalizing treatment to individual situations, the CYP450 genotyping test does have some drawbacks.

One major drawback is that the test can't actually predict if certain antidepressants will improve your depression symptoms. The test can only show which antidepressants probably won't work.

In addition, the test can be used only for certain antidepressants, not all of them. That's because the test is able to check only for two genes that are responsible for metabolizing certain antidepressants. Other genes affect how your body responds to other antidepressants, and the CYP450 test isn't able to check for them.

Here are the antidepressants the CYP450 test can be used for, with their generic names followed by available brand names in parentheses:

• Desipramine (Norpramin)
• Fluoxetine (Prozac, Prozac Weekly)
• Imipramine (Tofranil)
• Nortriptyline (Aventyl, Pamelor)
• Paroxetine (Paxil, Paxil CR)
• Venlafaxine (Effexor, Effexor XR)

These two genes may also influence your reaction to two other medications that haven't been FDA approved for use in depression but that are often used off-label to treat depression. These medications are:

• Clomipramine (Anafranil)
• Diazepam (Valium, Diazepam Intensol)

Finally, although the cytochrome P450 test has been FDA approved for certain situations, it may not be available in all communities yet. In addition, not all psychiatrists and other doctors routinely use the test.

Other factors also important in selecting antidepressants

If you opt for testing, keep in mind that the cytochrome P450 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 still important.