One in ten may be taking the wrong dose of this antidepressant
A new Norwegian study suggests that your genes could determine how well a commonly prescribed antidepressant works.
"The problem is that doctors often carry out testing after patients have already experienced difficulties," says Professor Espen Molden.(Photo: Diakonhjemmet Hospital)
Venlafaxine, better known by its brand name Efexor, is one of the most widely used medications for depression. Around 39,000 Norwegians used the medication in 2025, according to the Norwegian Prescribed Drug Registry.
Norwegian researchers have now discovered that our genes may determine whether this treatment works.
"We already knew that genes affect how much of the medication is present in the blood. What we've now demonstrated is that these differences also influence treatment outcomes," says Professor Espen Molden.
He is head of research at Diakonhjemmet Hospital's Centre for Psychopharmacology and led the study, which was published in The Lancet Psychiatry.
Venlafaxine is broken down in the liver by enzymes controlled by our genes. In some people, this breakdown happens very slowly, while in others it happens much more quickly, the researcher explains.
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A simple genetic test can predict how your body will tolerate this commonly used antidepressant.(Stock photo: Microstock)
More likely to switch medications
The researchers analysed data from more than 5,400 Norwegian patients who had taken venlafaxine between 2005 and 2025. They looked at which patients switched to a different antidepressant within one year.
Switching medications was used as an indication that the treatment had not been successful. This can happen either because of side effects or because the medication has no effect.
Around 1in 10 people metabolise venlafaxine at an unusual rate. In the study, around 7 per cent had slow metabolism, while about 2 per cent had rapid metabolism.
The researchers found that those with slow metabolism were about twice as likely to switch medications as patients with normal metabolism. This association was also confirmed in a separate group of more than 3,600 patients in the United Kingdom.
The patients with slow metabolism generally received slightly lower doses than usual from their doctors, but they were still more likely to switch medications. The researchers therefore suspect that even lower doses may be necessary to reduce the risk of side effects.
“If you don't get this kind of test, you'll receive the standard dose. And if your body is unable to break down the medication properly, the standard dose will effectively be an overdose,” says Molden.
Patients with rapid metabolism also switched medication more often in the Norwegian patient group, but this finding was not replicated in the British patient group.
The genetic test Molden is referring to costs between NOK 1,500 and 2,000 (about USD 158 to 211). The Centre for Psychopharmacology performs these tests, with approximately 10,000 conducted annually, according to Molden.
“The test result remains the same throughout your life. You take it once, and then you have the answer,” he says.
He emphasises that the test does not predict which illnesses you might develop, only how quickly your body breaks down medications.
Molden argues that genetic testing should ideally take place before patients begin treatment, rather than being introduced only after complications arise.
"The problem is that doctors often carry out testing after patients have already experienced difficulties. Our aim should be to prevent those problems in the first place, which means testing patients when treatment begins," he says.
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Molden believes genetic testing is not being used frequently enough, partly because doctors know too little about it.
"I would say it's not used enough, but you also have to make sure it's used appropriately," he says.
Wants more personalised treatment
Lars Lien, professor of psychiatry and head of the Norwegian Psychological Association, says it has long been known that our genes influence how medications work.
"This isn't new knowledge, but what I find exciting about this study is that the researchers have measured the relationship more directly and provided concrete figures," he says.
Lien believes the study shows that many patients receive treatment that does not work for them. He thinks this also applies to other medications, such as antibiotics.
"Many experience an incredible number of side effects without understanding why. If they'd undergone genetic testing, they might have had an explanation," he says.
He believes this kind of testing could help doctors select treatments that are better matched to each patient.
"The goal is to personalise treatment, rather than relying solely on what we know about how medications work across large groups of patients," says Lien.
Lien also wants more research. He would like to see the findings replicated in additional studies and similar investigations carried out with other medications.
Lars Lien is a professor of psychiatry and head of the Norwegian Psychological Association.(Photo: Lars Bjarne Mythen / INN)
Not tested in practice
The study has some limitations. The researchers cannot determine whether patients switched medications because of side effects or because the medication was ineffective.
Whether genetic testing before starting treatment actually leads to better treatment outcomes needs to be investigated further in new studies, according to the Norwegian researchers.
Molden also points out that all patients would need to be tested to identify the approximately 10 per cent who metabolise the medication abnormally.
"It’s a question of healthcare costs that deserves serious discussion," he says.
