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New Genetic Tests Help Doctors Prescribe the Right Medicine for You

Alina Eisen, Pharm.D Candidate 2009

University at Buffalo School of Pharmacy & Pharmaceutical Sciences

Prepared during Consumer Health Information Corporation Clerkship


Dorothy L. Smith, Pharm.D.

CEO and President

Consumer Health Information Corporation

McLean, Virginia

In December 2008, the New York Times ran a story about a woman from California who had been taking a drug called tamoxifen to help prevent breast cancer.  After two years of taking the drug, her doctor ordered a new genetic test that showed that her genes were preventing the drug from working properly.

“You find out you’ve been taking this medication for all this time, and find out you are not getting benefit…I was devastated” says the women.  She had to stop taking tamoxifen.  The good news is that she found out that the drug was not helping her and her doctor can now prescribe a drug which will work in her body.  The bad news is that she could have known this two years ago if she had taken the genetic test from Day 1. 

Experts report that approximately $300 billion is wasted each year on drugs which apparently do not work in people who have certain genes. These people never receive the full benefit from these drugs.  Others are getting dangerous side effects. 

For example, the blood-thinning drug warfarin is one of the top twenty drugs prescribed in the US.  It is used to help prevent blood clots.  If a person’s genes prevent the drug from working correctly, warfarin becomes dangerous. It is one of the top three drugs that cause hospitalization or emergency room visits. If a person has genes that allow too much warfarin to get into the bloodstream, the blood cannot clot correctly and the person can have bleeding.  On the other hand, if a person has genes that prevent enough warfarin from getting into the bloodstream, the person could develop serious blood clots.  The way a person’s body reacts to warfarin, tamoxifen and other drugs depends on differences in their genetic makeup.


How do genes affect the way drugs work?

Genes provide your body with instructions for making enzymes.  Enzymes are needed for your body to break down drugs so your body can get benefit from the medicine.  You carry two copies of every gene: one from your mother and one from your father.  Differences in these genes can affect the speed of different enzymes you have in your body.  This affects how well your body is able to use medicines and how well drugs work in your body.  Differences in your enzymes can affect how your body can metabolize (break down) a drug and how long the drug stays your body.  Based on what type of genes you carry, you may be:

  • a poor drug metabolizer
    If you are a “poor metabolizer”, you do not break down drugs well.  This may result in too much drug in the body which may lead to a dangerous side effect or even death.  In some cases, your body may not be able to break down certain drugs to their working form and therefore the drugs will not work properly.
  • an extensive or “normal” drug metbolizer
    You metabolize drugs at the normal rate.
  • an ultra-rapid drug metabolizer
    If you are an “ultra-rapid” metabolizer, this means you break down drugs too fast, causing them to be of no use in the body.  If medications do not work properly, conditions such as high blood pressure, blood disorders, and cancer will be left untreated and may even lead to death.

Genetic Tests for Drug Response

Researchers have now found more than 30 types of drug metabolizing enzymes in humans and mostly all of them vary between people. Your doctor can now give you a blood test to determine the effect these enzymes have on medications. 

The three main genetic tests available today include: CYP2D6, CYP2C9, and CYP2C1. 

Name of Enzyme Test             Common drugs that are broken down by enzyme     


CYP2D6                                                      Antidepressants (Paxil, Prozac)                                          

                                                                     Antipsychotics (Abilify, Risperdal)           

Affects 25% of drugs                             Antihypertensives (Torpol XL)

                                                                     Antiarrhythmics (Rythmol)

                                                                     Pain Killers (Codiene)

                                                                     Breast Cancer (Tamoxifen)


 CYP2C9                                                     Warfarin (Coumadin)

                                                                     Glipizide (Glucotrol)

 Affects 15% of drugs                             Ibuprofen (Motrin, Advil)




                                                                      Lansoprazole (Prevacid)

Affects 5 -10% of drugs                          Omeprazole (Prilosec)

                                                                      Diazepam (Valium)

                                                                      Phenytoin (Dilantin

What about the cost?

Genetic tests for drug response usually cost a few hundred dollars.  The cost of having a genetic test done should be weighed against the cost-savings and benefits that might result.  If you know the results of your genetic test before you start taking a medicine, your doctor will know which drug is best for you.  You will not waste money purchasing medicines that the test shows will not work or cause dangerous side effects.  This could be life-saving in many diseases.   If you weigh the cost of the test against the costs you would face if you developed a serious side effect or took a drug that did not work, the cost of taking a test would be far less than the cost of treating complications. 

Getting tested is a once in-a-lifetime experience because your genes do not change over time.  Since many drugs are metabolized by one enzyme, you may only need one test.  Once you are tested, you can keep your genetic test results for the rest of your life and share them with future care professionals.  

Some insurance companies may cover genetic testing depending on your policy and your reasons for testing.  If you are interested in getting tested, you can call your insurance company to find out the cost and requirements for testing.  Your doctor will have to order the test for you.


Patient Information Tips: How can genetic tests help me?


Are you wondering if the drug you were prescribed is working for you?  Or may it be causing dangerous side effects?  The Food and Drug Administration now requires certain dangerous drugs to have genetic testing information in their labeling.  Some doctors are now having their patients take a genetic test before prescribing certain medications. You should ask your doctor or pharmacist if genetic testing is right for you.


The results of these genetic tests can help you and your doctor:

  • Adjust the dose of a medication so that it is the “right dose” for you. “Poor metabolizers” might need to take a lower dose because they break down drugs slowly. “Ultra-rapid metabolizers” might need a higher dose because they break down drugs too fast.
  • Prescribe medications that will work properly in your body and help you to get full benefit of the drug. 
  • Avoid medications that may be more likely to cause dangerous side effects in you.
  • Save you money by getting the most accurate treatment right from the start.


Personalized medicine in the future?

It will probably take several years or even decades until you can start carrying a key chain or a “DNA chip” containing your genetic makeup and personal health information.  However, regular genetic testing may be part of routine doctor visits someday soon.  More research is still needed to find out whether knowing your genetic makeup before prescribing you a drug will allow drugs to work better or prevent adverse drug reactions.

In the future, drug companies will be able to use this information to design new “personalized” medicines that are more likely to work in your body.  Genetic testing may also help you save money in the long run. Because the cost of taking a drug that shows no benefit in you is most likely going to exceed the cost of getting a genetic test.  Remember, lost money can be recovered, however lost years cannot.


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