Your genes could change your prescription. So why aren't doctors testing?

One in four people has been prescribed a drug for which their genetics could affect treatment, according to a recent study. Genetic testing could help doctors choose safer, more effective drugs, but cost and other barriers currently limit its routine use in Canada, researchers say.

New study finds nearly 1 in 4 participants had taken medication potentially affected by their genes

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One of the clues your doctor may be missing when prescribing medication is in your DNA.

A study published Wednesday in Science Translational Medicine found that nearly one in four participants had been prescribed a medication that could have required a different drug or dose based on their genetics.

The study, led by researchers at the University of Pittsburgh, examined 16 genes that affect how the body responds to medications.

DNA samples from 98,590 people in the National Institutes of Health's All of Us Research Program were analyzed and linked to participants' medical records when available.

Each carried at least one relevant genetic variation that could affect how their body responds to certain medications, with a typical participant carrying nine. Among those with both genetic and clinical data, 24.4 per cent had been prescribed at least one medication that, based on their genes, guidelines suggest should have been replaced with another drug or given a different dose.

However, the study did not determine whether patients experienced side effects or whether their medications were less effective because of their genetic makeup.

The study's senior author, Dr. Philip Empey, an associate professor at the University of Pittsburgh school of pharmacy, says knowing a patient's genetic profile could help reduce the trial and error of medicine.

"I think anything we can do to better predict the first time, what the best medicines should be, what the best dose should be, the more we can reduce that trial and error," he said.

A pharmacist sorts medications at a pharmacy in Vancouver in October 2022. Genetic variations can affect how patients respond to certain medications, potentially requiring doctors to adjust the dose or prescribe an alternative drug. (Darryl Dyck/The Canadian Press)

Finding the right medication

Omeprazole and pantoprazole for acid reflux and atorvastatin for cholesterol were among the most common medications prescribed to participants with genetic profiles that could call for changes in treatment.

Empey says some genetic variations can increase the risk of muscle problems when patients take statins like atorvastatin, which are used to reduce the risk of heart attack or stroke.

This knowledge could help doctors choose another drug or to adjust treatment.

"It allows the clinicians to make better decisions, to say, 'Well, maybe not that cholesterol medicine. Maybe we start slower. Maybe there's a maximum dose. Maybe we watch for specific toxicities more aggressively,'" Empey said.

For some medications, the consequences can be more serious.

For example, experts interviewed by CBC News say genetic testing can help identify patients who are unlikely to respond well to clopidogrel (Plavix), a drug used to prevent blood clots after cardiac stenting, allowing doctors to treat them with a different medication.

Dr. James Kennedy is head of the Tanenbaum Centre for Pharmacogenetics at Toronto's Centre for Addiction and Mental Health. He says genetic testing can help doctors choose medications, but other factors, including diet, lifestyle and medications a patient is already taking, must also be considered. (CBC)

A blueprint, not a guarantee

The study's results were not surprising, says Dr. James Kennedy, head of the Tanenbaum Centre for Pharmacogenetics at Toronto's Centre for Addiction and Mental Health (CAMH), but he was particularly impressed by its genetic diversity and representation of people of African ancestry.

Kennedy says his team has provided genetic testing for approximately 11,000 patients, many of whom had already tried two or three antidepressants without satisfactory results.

About eight to 10 per cent of people break down certain commonly prescribed antidepressants unusually slowly, allowing the medication to accumulate and potentially cause side-effects, he says.

Kennedy says genetic testing is especially useful for patients who may have already struggled with several medications.

However, DNA testing alone will not predict how a patient will respond to a given medication, he said. Other medications the patient is taking, whether they smoke and their diet must also be taken into consideration.

"We say, 'OK, here's your blueprint, here's some tendencies you have, and now here's a discussion of all the other things you have to consider before you make a final decision,'" he said.

A 2023 European clinical trial cited in the study found that genetic testing before prescribing certain medications reduced adverse drug reactions. But making such testing routine in Canada remains a challenge.

A researcher at the University of Calgary holds a diagnostic kit used to identify genetic variations in February 2024. While such testing could help doctors tailor medications to individual patients, cost and practical challenges have limited its widespread use in Canada. (Jeff McIntosh/The Canadian Press)

Why Canada isn't testing everyone

Toronto cardiologist Dr. Jacob Udell says the study was impressive, particularly given the genetic diversity of its participants, but few clinicians in Canada or the U.S. routinely screen patients for genetic variations to understand how they might metabolize medications.

"It's certainly not on the front-line clinicians' radar to do this," he said.

Udell says that for many conditions, a doctor can prescribe a medication, monitor the patient's response and make adjustments as necessary, usually without serious consequences. That approach can be more practical than routinely screening patients for genetic variations.

He also questions whether widespread testing should be a priority when Canada's health-care system is struggling to provide more basic services.

"We're just trying to get everyone a family doctor," he said.

In addition to the costs of the tests, Udell says clinicians would need access to the results of genetic tests and the expertise to interpret them.

Empey agreed that making genetic testing routine would require significant investment in training, infrastructure and new systems to share results between health-care providers.

"The science is strong for many medicines. The question about implementation then becomes more economic, more process-oriented and more education-oriented," he said.

For now, Empey says patients can ask their doctor or pharmacist whether genetic testing would be useful and share any results with them.

Udell warned that private testing may become more common for patients who can afford to pay for it, "but that won't help those who can't afford the testing."

ABOUT THE AUTHOR

Eric Michael Johnson is a senior writer and editor based in Vancouver. Before joining CBC, he was an award-winning science writer and editor. He holds a PhD in the history of science from the University of British Columbia. He can be reached at eric.johnson@cbc.ca.