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A test that shows how your body might react to medication

What if there was a way to understand how you may react to medicine before you even took one pill?

Genetic screening that reveals how your body may respond to prescription drugs makes this possible - and Dr. Darren Larsen, MD, has seen the potential life-saving impact of pharmacogenomics firsthand. 

“Not only did it help him, it may have saved him,” Dr. Larsen says of the knowledge gained from pharmacogenomics  for a young man who had a history of depression and was actively suicidal. “The screening revealed that there was one drug that was likely to be most effective in treating his depression - an older drug that no one had thought to use prior to this point. I prescribed it after seeing the results of pharmacogenomic testing. He started feeling better three weeks later, and three months later, he was back at university living a full life.”

Examples like this have happened multiple times in Dr. Larsen’s practice. We caught up with him to learn more about pharmacogenomics, and how the results of this test can be beneficial.

What is pharmacogenomics?

Pharmacogenomics allows people to understand what type of prescription medication might be most effective for them based on their unique genetic makeup. It can also provide insights into how an individual may react to over the counter medications, such as Tylenol or Advil, and even some supplements.

“Generally, the body disposes of medicine either through your liver or through your kidneys,” Dr. Larsen explains. “In your liver, you have a series of enzymes that transform or break down all kinds of chemicals - drugs being some of those. In some cases, an individual’s liver enzymes will allow that person to break down medicine just fine. Others may have a slower metabolism and don’t break down medications as quickly. In yet other cases, some people’s enzymes may break up the drug too fast - it gets destroyed so quickly that it won’t have the desired effect.”

Dr. Larsen points out that genes associated with liver enzymes are hereditary, which means we can use pharmacogenomics to learn more about how individuals may process medication.

“Your genes never change,” he says. “But the more we can learn about these liver pathways, the more opportunities we have to get a prescription correct right off the bat.” 

And the testing process itself, Dr. Larsen says, is relatively straightforward.

How pharmacogenomics works

Collecting a DNA sample for pharmacogenomic screening can be done easily. If you’re visiting TELUS Health Care Centres in person, a cheek swab can be done at the time of your visit. If you’re doing the test from home, you’ll be sent a kit that allows you to take a cheek swab (a q-tip-like tool). It collects cells from the inside of your cheek, and is then mailed back to the lab.

Once the results are in, you’ll get a detailed report that you can share with your healthcare providers as well as a personalized summary of your results written by our TELUS Health genetic counsellor. The report may be a helpful source of information for conversations with your family doctor and other healthcare providers in the future.

“Read the report carefully,” encourages Dr. Larsen for anyone taking the test. “And ask our genetic counsellor questions about anything you don’t understand. Then, share it with your family doctor. The results of the test are most useful at the time that your physician is deciding on appropriate medication for you, if the medication is one that is metabolized in the liver."

Another thing Dr. Larsen recommends is sharing the results with your family - particularly your parents and adult children. “One of them may have a similar outcome,” he says. “This is particularly true when it comes to mental health, since mental illness can have a hereditary component. You can’t predict your metabolism directly - in other words, everyone will have to get their own test to know for sure what their own results are - but there could be some overlap or similarities between family members.”

The benefits of pharmacogenomics

According to Dr. Larsen, there are four primary benefits that may be seen, and are all intertwined, when it comes to pharmacogenomics:

  • Potential for faster treatment and feeling better sooner;
  • Reduced potential for harmful side effects;
  • Fewer trials of different medications, which may lower cost spent in time and money; and
  • Improvement of health issues at the lowest effective dose for many prescription medications.

“The test may allow doctors to prescribe a drug that’s more likely to be effective right from the start,” Dr. Larsen says. “There’s a lot of research that has been done particularly when it comes to medicines prescribed for mental illness. Beyond mental health, pain medications, such as opioids and anti-inflammatories, as well as cardiovascular drugs and more complex antibiotics are all examples of medications that pharmacogenomics results can impact.  We can better predict which type will work best for an individual patient.”

Lowering the potential trial and error to find a medication that works for a patient can also have economic benefits too. “The biggest reason, in my experience, of someone never starting a prescription, is because they can’t afford it,” shares Dr. Larsen. “Pharmacogenomics may allow us to remove a trial and error phase, and find lower cost alternatives from the get-go.” 

The most common reason that people discontinue a drug, he says, is because of negative side effects - something that can also be mitigated by prescribing drugs that a doctor knows will align with an individual’s particular liver enzymes.

“This test can be valuable for any individual,” Dr. Larsen explains. “Even if you’re only taking medication for a short period of time, it’s still important to know the likelihood that it will work and have the fewest side effects possible. If you’re on medications long term, this becomes even more important.”

A personalized approach to prescriptions

What inspires Dr. Larsen the most about the possibilities that pharmacogenomics offers is a truly personalized approach to prescribing. It’s a way for doctors to remove some of the guesswork out of deciding on a prescription, and for a patient to know they are getting optimal personalized care when it comes to their medication options.

“If you are taking any medicine long term and you want to ensure it’s the lowest risk, has the greatest potential benefit and is the most cost effective, this test is the best possible method we have for making that happen,” he says.

Pharmacogenomics is currently available at TELUS Health Care Centre clinics in Toronto, Ottawa, Vancouver and Calgary. For more information, visit

Written in consultation with Dr. Darren Larsen, MD.