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Pharmacogenetics in medical practice: from indication to treatment decision

Reviewed by Dr. Daniel Wallerstorfer, molecular biologist and founder of Novogenia · Updated: 13 October 2026 · Reading time: 9 min

A genetic test at the crossroads

What happens when a genetic test helps decide which drug a patient gets after a heart attack? That is exactly what the Dutch POPular Genetics trial set out to answer. 2,488 patients received a stent during primary PCI – the emergency catheter procedure for an acute heart attack. Afterward, all of them needed an antiplatelet drug. Half received ticagrelor or prasugrel as standard. For the other half, an early CYP2C19 genetic test made the call: anyone carrying a gene copy that makes the body activate clopidogrel less well (CYP2C19*2 or *3) got ticagrelor or prasugrel. Everyone else got clopidogrel.

After twelve months, the genotype-guided strategy did no worse on serious events – death, heart attack, stent thrombosis, stroke or major bleeding: 5.1% versus 5.9%. And there was less bleeding: 9.8% versus 12.5% (Claassens et al., New England Journal of Medicine, 2019).

The logical conclusion: the genotype helps you pick, from several good options, the one that suits this particular person. Think of the result as an extra paragraph in the package insert – written for one patient only. You still have to read it, weigh it and decide. Here is how pharmacogenetics fits into your practice, step by step.

When testing makes sense: typical triggers

Pharmacogenetics is not an everyday test, but it fits many situations you know from practice:

• Before a drug with a clear gene-based recommendation: for example before fluoropyrimidines such as 5-fluorouracil or capecitabine (gene DPYD), or before clopidogrel after acute coronary syndrome or PCI (gene CYP2C19). Which genes affect which drug groups is covered in Key genes: CYP2D6, CYP2C19 and more.
• When a drug does not work as expected: no effect at a normal dose, or unusually strong side effects even at a low dose.
• When many drugs are taken at once: as part of a medication review, a result shows where genes play a role.
• Ahead of time: a panel before any medication is due. When that makes sense is covered in Pre-emptive or reactive testing?

A study from 17 Dutch hospitals shows how well targeted testing works in everyday care. Before chemotherapy with fluoropyrimidines, 1,103 patients were tested for four DPYD variants. 85 of them (8%) carried such a variant and received a reduced starting dose. For carriers of DPYD*2A, the risk of severe side effects compared with people without a variant was 1.31-fold – versus 2.87-fold in an earlier comparison group treated at full dose. Upfront testing proved easy to run in routine practice (Henricks et al., Lancet Oncology, 2018).

International guidelines build on exactly this: for DPYD intermediate metabolizers (activity score 1 or 1.5), the Clinical Pharmacogenetics Implementation Consortium (CPIC) recommends reducing the starting dose by 50% and then adjusting it based on tolerability; for poor metabolizers, CPIC advises avoiding fluoropyrimidines. How CPIC, DPWG and drug labels are structured is explained in Guidelines: CPIC, DPWG and drug labels.

The process in six steps

From the first question to the entry in the patient record, there are six steps. You know most of them from any other lab test.

Steps 1 and 2: the question first, then the test

A good test starts with a good question. These points get you to the core quickly:

• Which drugs is your patient taking today, and which are planned?
• Have any drugs failed to work or caused unexpectedly strong side effects?
• Is there a gene-based recommendation for the planned drug in CPIC, DPWG or the drug label?
• Is a single gene enough, or would a panel that also covers later prescriptions make more sense?

Next comes informing the patient. The rules differ markedly between Germany, Austria and Switzerland:

• Germany: Pharmacogenetic testing is a diagnostic genetic examination (Section 3 No. 7(c) GenDG) that only physicians may perform (Section 7(1)). Beforehand, you explain its nature, significance and implications and allow a reasonable time to decide (Section 9). Consent must be explicit and in writing; it also covers whether, and to what extent, the result is disclosed or destroyed (Section 8(1)). Once the result is in, genetic counseling should be offered (Section 10(1)).
• Austria: The Gene Technology Act does not mention pharmacogenetics explicitly in Section 65. Section 68(1) GTG requires a specialist referral only for analyses of disease predispositions (types 3 and 4).
• Switzerland: Pharmacogenetic testing is a test in the medical field (Art. 19 GUMG). Physicians may order it even without a postgraduate title in the relevant specialty if a defined selection of variants is tested, results of known clinical significance are expected and measures are available (Art. 5 GUMV).

The details are covered in Law: who may offer pharmacogenetics? and Data protection and consent.

Steps 3 and 4: a little saliva, a lot of information

A saliva sample is all the analysis needs. Make sure your patient follows the instructions in the kit and that the sample is clearly assigned to them. In the lab, the DNA is extracted and the genetic variants are determined.

Before you use a result, three questions to the lab are worth asking:

• What was read? Which genes and which variants the test covers. An unremarkable result only means that none of the tested variants was found.
• Are gene copies captured? For CYP2D6, missing or duplicated gene copies count too – they can switch a tool off completely or make it work double time.
• How is the result translated? Which system the lab uses to derive a phenotype from the genotype, for example the activity score from the CPIC and DPWG guidelines.

For a reactive test, allow time for shipping and analysis so the result is in before treatment starts.

Steps 5 and 6: the result in medication management

A pharmacogenetic result reads in three layers: genotype, phenotype, recommendation. “CYP2C19 *1/*2” becomes “intermediate metabolizer,” and for clopidogrel after acute coronary syndrome or PCI, this leads to the CPIC recommendation to avoid the standard dose if possible and – if there is no contraindication – to use prasugrel or ticagrelor instead. How to read and explain these layers with confidence is covered in Understanding and communicating results.

A result unfolds its full value when it does not disappear into a drawer:

• File it permanently: Enter the gene, genotype and phenotype in the record like an allergy – visible with every new prescription.
• Check it against the medication list: Compare current medication with the result and decide when to review it.
• See the whole picture: Age, kidney and liver function, and other drugs that slow down the same enzyme change the effect as well.
• Share it where it helps: The pharmacy and co-treating physicians benefit from the result. In Germany, only with explicit consent (Section 11(3) GenDG). How pharmacies can contribute is described in Pharmacogenetics in the pharmacy.

The decision stays with the physician

The result does not replace a medical decision – it gives that decision a firmer footing. Whether you adjust a dose, choose a different drug or leave everything as it is, you decide together with your patient. Genes are one factor among many. Recommendations derived from a genetic profile are logical conclusions – the guidelines show how well founded they are for each drug.

Tell your patient clearly: they should never stop a drug or change its dose on their own because of the result. Decisions about medication – stopping, switching or changing the dose – are made by the treating physician. Your patient can repeat this sentence at the pharmacy and to other physicians, too.

Checklist for your practice

• The question is clear: which drug, which gene, reactive test or panel.
• The legal situation in your country is checked, the patient is informed and consent is documented.
• The saliva sample has been taken according to the instructions and clearly assigned.
• You know which genes and variants the test covers and how the lab derives the phenotype.
• The result has been checked against guidelines and the overall clinical picture.
• The decision has been discussed with the patient and documented with its rationale.
• The result is filed permanently and visible with new prescriptions.
• Your patient knows: no change to medication without the treating physician.

Frequently asked questions

Related: Understanding and communicating results · Law: who may offer pharmacogenetics? · Offer for physicians

Note: This guide is general information for businesses and professionals and does not replace medical or legal advice. The genetic traits determined in a genetic analysis are well documented (each gene included is backed by at least three independent studies); recommendations derived from them are logical conclusions, not scientifically proven. Pharmacogenetic results do not replace a medical decision: any change to medication is made only by the treating physician. Regulations can change; errors and changes excepted.