Preventing Adverse Drug Reactions with Pharmacogenetic Testing

Every year, hundreds of thousands of people in the UK and across the world are hospitalized because of unexpected side effects from medications they were prescribed. These aren’t allergies or overdoses - they’re adverse drug reactions (ADRs), often caused by how a person’s genes process drugs. For many, it’s not bad luck. It’s biology. And now, there’s a way to see it coming before the first pill is even taken.

What Pharmacogenetic Testing Actually Does

Pharmacogenetic testing looks at your DNA to find out how your body breaks down medicines. It’s not about predicting disease. It’s about predicting how you’ll respond to a drug. Some people metabolize drugs too fast - the medicine leaves their system before it can work. Others metabolize too slow - the drug builds up and causes toxicity. Your genes decide which group you’re in.

Take clopidogrel, a common blood thinner after a heart attack. About 30% of people have a variant in the CYP2C19 gene that makes this drug useless for them. Without testing, they’re left with a false sense of security, risking a second heart attack. With testing, doctors switch them to a different drug - like prasugrel or ticagrelor - and avoid the danger entirely.

Or consider codeine. It’s a painkiller that turns into morphine in the body. But some people have extra copies of the CYP2D6 gene. Their bodies convert codeine into morphine too quickly. In rare cases, this has led to fatal breathing problems in children after tonsil surgery. Testing can spot these ultra-rapid metabolizers before the prescription is written.

The Landmark Study That Changed Everything

In 2023, a massive study called PREPARE - led by researchers at the University of Liverpool and involving nearly 7,000 patients across seven European countries - proved this isn’t just theory. It’s practice.

Patients were tested for 12 key genes before being prescribed any of over 100 common medications. These included genes like CYP2D6, CYP2C19, TPMT, SLCO1B1, and HLA-B. The results? A 30% drop in serious adverse drug reactions. That’s not a small improvement. That’s a game-changer.

What made this study different? It wasn’t done after someone got sick. It was done before any drugs were given. That’s called preemptive testing. And it works better than waiting for a reaction to happen - reactive testing only cuts ADRs by 15-20%.

Which Genes Matter Most

Not every gene matters for every drug. But for common prescriptions, a handful make all the difference:

• CYP2C19: Affects clopidogrel, antidepressants like citalopram, and proton pump inhibitors like omeprazole.
• CYP2D6: Impacts codeine, tramadol, tamoxifen, and many antidepressants.
• TPMT: Critical for azathioprine and mercaptopurine - used in autoimmune diseases and cancer. Without testing, patients can develop life-threatening bone marrow suppression.
• SLCO1B1: Predicts muscle pain from statins like simvastatin. A simple test can help avoid a painful, dangerous side effect.
• HLA-B*1502: A genetic marker that, if present, makes carbamazepine (used for epilepsy and bipolar disorder) extremely risky for people of Asian descent. Testing cuts the risk of Stevens-Johnson syndrome by 95%.

These aren’t obscure genes. They’re found in everyday prescriptions. And the data is clear: if you’re taking one of these drugs, your genes could be silently working against you.

How It Works in Real Clinics

Getting tested isn’t complicated. A simple cheek swab or blood sample is sent to a lab. Results come back in 24 to 72 hours. In hospitals like those in the UK’s NHS, results are now integrated into electronic health records. When a doctor prescribes a drug like clopidogrel or simvastatin, a pop-up alert appears: "Patient has CYP2C19 poor metabolizer variant. Consider alternative therapy."

This isn’t science fiction. It’s happening now. The University of Florida’s personalized medicine program has been doing this since 2012. They saw a 75% drop in ADR-related emergency visits among tested patients. The cost? Around $1.2 million to set up. The payback? Less than two years, thanks to fewer hospitalizations and fewer wasted prescriptions.

In the UK, the NHS estimates ADRs cost £500 million a year in avoidable hospital stays. Pharmacogenetic testing could cut that significantly. And with the European Commission committing €150 million to roll this out by 2027, it’s only going to become more common.

Why It’s Not Everywhere Yet

Despite the evidence, adoption is still slow - especially in primary care. Only 18% of GP practices in the UK have integrated pharmacogenetic testing. Why?

First, many doctors don’t feel confident interpreting the results. A 2022 survey found only 37% of physicians felt comfortable using genetic data to adjust prescriptions. Training helps. A few hours of CME-accredited education can change that.

Second, there’s confusion over what to do with "intermediate metabolizers" - people who fall between fast and slow. Guidelines from the Clinical Pharmacogenetics Implementation Consortium (CPIC) help, but they’re not always easy to apply in a 10-minute appointment.

Third, cost. A full panel test runs between £150 and £300 in the UK. That’s not cheap. But compared to the £10,000 average cost of an ADR-related hospital admission, it’s a bargain. And the price is dropping. New point-of-care tests are being developed that could bring the cost under £50 by 2026.

Who Benefits the Most

Some groups see the biggest gains:

• People on multiple medications: Polypharmacy increases ADR risk. Testing helps untangle which drug might be causing trouble.
• Patients with psychiatric conditions: Antidepressants and antipsychotics have high rates of side effects. One study showed a 40% reduction in side effects after genotype-guided prescribing.
• Cancer patients: Chemotherapy drugs like 5-FU and irinotecan can be deadly if metabolized poorly. Testing prevents severe toxicity in up to 10% of patients.
• Older adults: As liver and kidney function decline with age, genetic factors become even more important in drug dosing.

And here’s something surprising: 93.5% of people tested in the PREPARE study had at least one gene variant that affected how they responded to a drug. That means almost everyone has something to gain.

What’s Next

The future isn’t just about single genes. Researchers are now building polygenic risk scores - combining dozens of genetic markers to predict drug response with even greater accuracy. Early results show a 40-60% improvement over single-gene testing.

The FDA has added 42 new gene-drug pairs to its list since 2022, bringing the total to 329. The European Medicines Agency now includes pharmacogenetic warnings in nearly a third of new drug labels.

And the public? Most people are ready. In surveys, 85% say they’d be willing to get tested if their doctor recommended it. The big worry? Privacy. About one in three people fear their genetic data could be misused. That’s why secure, anonymized systems and strict data governance are just as important as the science.

Is It Right for You?

You don’t need to get tested for every drug. But if you’re:

• On long-term medication (especially for depression, epilepsy, heart disease, or cancer)
• Experiencing unexplained side effects
• Have family members who had bad reactions to the same drug
• Or are about to start a new high-risk medication

…then ask your doctor about pharmacogenetic testing. It’s not magic. But it’s one of the most powerful tools we have to make medicine safer - and more personal.