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Genetic Variation and Antidepressant Induced Side Effects

Ruslan Dorfman, PhD, MBA1 Nov 2026

Depression affects not only the person living with it but also their family and friends. Antidepressants help many people feel better, but side effects are a major reason people stop taking them, often before the medication has had a chance to work. Some of these side effects are linked to your genes, which affect how quickly your body breaks down many antidepressants. Here is what research shows about antidepressant side effects, which genes matter, and what genetic testing can and cannot tell you.

How common are antidepressant side effects?

Very common. In a US study of 401 people starting a selective serotonin reuptake inhibitor (SSRI), 86% reported at least one side effect, and 55% had at least one they found bothersome. Most side effects began in the first two weeks, and many were still present three months later. The study also found that doctors tended to underestimate how often side effects happen and how much they bother patients.

At the same time, not every symptom is caused by the medication. In the GENDEP study of 811 adults with depression, many complaints often blamed on antidepressants, such as tiredness, poor sleep and headaches, were actually more common before treatment started. Depression itself causes many physical symptoms, so it helps to note how you felt before starting a medication. Read more about understanding and reducing antidepressant side effects.

Which side effects are most common?

Side effects depend on the type of antidepressant:

SSRIs and SNRIs (for example citalopram, escitalopram, sertraline, fluoxetine, paroxetine, venlafaxine and duloxetine) often cause nausea, diarrhea, sleep problems, drowsiness, sweating and sexual problems. In the US study, sexual problems and drowsiness were the most common bothersome side effects (17% each), and sexual problems usually did not go away on their own. Read more about antidepressants and sexual health.
Tricyclic antidepressants (for example amitriptyline and nortriptyline) more often cause dry mouth, constipation, drowsiness, dizziness and weight gain. In GENDEP, dry mouth affected 74% of people taking nortriptyline, constipation 33% and weight gain 15%.

Rarely, combining medications that raise serotonin can cause a dangerous reaction called serotonin syndrome. Health Canada and the US Food and Drug Administration also warn that antidepressants can increase suicidal thoughts in some people, especially young people, early in treatment or after a dose change. Contact your doctor right away if your mood worsens or you have thoughts of self-harm.

How do genes affect antidepressant side effects?

The clearest genetic link is through drug metabolism. Liver enzymes, mainly CYP2C19 and CYP2D6, break down most antidepressants. Your genes determine how active these enzymes are:

Poor metabolizers break the medication down slowly, so it builds up in the blood. This raises the chance of side effects at a standard dose.
Ultrarapid metabolizers clear the medication quickly, so blood levels may be too low for the medication to work.

The Clinical Pharmacogenetics Implementation Consortium (CPIC) has published guidelines that turn these results into prescribing advice:

CYP2C19 guides citalopram, escitalopram and sertraline. For example, CYP2C19 poor metabolizers may need a lower starting dose of escitalopram or a different medication, while ultrarapid metabolizers may do better with an antidepressant that does not depend on CYP2C19.
CYP2D6 guides paroxetine, fluvoxamine, vortioxetine and venlafaxine, as well as tricyclic antidepressants such as nortriptyline and amitriptyline. Poor metabolizers may need a lower dose or a different medication.
CYP2B6 also affects sertraline levels.

For tricyclic antidepressants, CPIC recommends avoiding them in CYP2D6 poor and ultrarapid metabolizers, or using a lower dose with blood level monitoring if they are needed. Learn more in Can genetic testing help me to choose psychiatric medications?

What about serotonin genes such as HTR2C?

Researchers have also studied genes for serotonin receptors and the serotonin transporter, which is the protein that SSRIs block. In GENDEP, a variant in the HTR2C gene, which makes the serotonin 2C receptor, was linked to more serotonin-related side effects such as nausea, diarrhea, poor appetite, sleep problems and sexual problems. However, the finding was not confirmed in a second large study (GenPod).

CPIC's 2023 guideline reviewed the serotonin transporter gene (SLC6A4) and the serotonin 2A receptor gene (HTR2A) and concluded that current evidence does not support using them to choose or dose antidepressants. No guideline recommends testing HTR2C before starting an antidepressant. For now, the metabolism genes CYP2C19, CYP2D6 and CYP2B6 are the ones with clinical recommendations.

Does genetic testing reduce side effects and improve treatment?

Genetic testing cannot predict every side effect, but it helps avoid medications and doses that are likely to cause problems based on how your body processes them. Studies show benefits for people with depression:

A meta-analysis of 13 trials found that people whose treatment was guided by pharmacogenomic testing were about 41% more likely to reach remission than those receiving usual treatment.
In the PRIME Care trial of 1,944 US veterans, clinicians who had test results chose an antidepressant with no predicted drug-gene interaction more often, and patients' remission rates over 24 weeks were higher.
In a Canadian trial in community pharmacies using Pillcheck, people whose treatment was guided by the test improved about twice as much in depression, anxiety and disability over six months as those receiving usual care.

Finding a well-tolerated medication sooner makes it easier to stay on treatment long enough for it to work. Read about genes and other psychiatric medications, such as antipsychotics and mood stabilizers.

What can I do if side effects bother me?

Do not stop suddenly. Stopping an antidepressant abruptly can cause withdrawal symptoms such as dizziness, irritability and flu-like feelings, and depression can return. Talk to your prescriber about the safest way to change, and read when and how to taper antidepressants.
Talk to your prescriber or pharmacist. Some side effects, such as nausea, often ease in the first few weeks. Others can be managed by changing the dose, the time of day you take the medication, or the medication itself.
Ask about a pharmacist medication review. Other medications and supplements can change antidepressant levels. A pharmacist can check your genes and all your medications together.
Consider pharmacogenetic testing, especially if you have had side effects or no benefit from one or more antidepressants.

A Pillcheck test reports CYP2C19, CYP2D6 and CYP2B6, and a pharmacist reviews your results together with your current medications.

Pillcheck testing is performed in CAP- and COLA-accredited clinical genetic laboratories located in Canada. Pillcheck reports on 24 genes, increasing coverage with new guidelines and drug labels (see the genes tab), and covers 355 medications, which account for over 70% of dispensed prescriptions. The Pillcheck test panel ensures high accuracy for people of non-European and admixed ancestries. Your results last a lifetime, and report updates are free of charge.

Summary

Most people taking an SSRI report at least one side effect, and about half find one bothersome. Sexual problems and drowsiness are among the most bothersome.
Some symptoms blamed on antidepressants are caused by depression itself.
CYP2C19, CYP2D6 and CYP2B6 affect how quickly your body breaks down many antidepressants. CPIC guidelines use these results to guide the choice and dose of medication.
Serotonin genes such as HTR2C, HTR2A and SLC6A4 have been studied, but they are not recommended for choosing antidepressants today.
Pharmacogenomic-guided treatment improved remission in clinical trials. Never stop an antidepressant suddenly; talk to your prescriber first.

Frequently asked questions

Will antidepressant side effects go away?

Some do. Nausea and stomach upset often ease within the first few weeks. Others, such as sexual problems, tend to continue. If a side effect bothers you, talk to your prescriber; there are often ways to manage it.

Can a genetic test predict which side effects I will get?

Not exactly. A pharmacogenetic test shows how quickly your body breaks down many antidepressants, which helps your prescriber avoid doses and medications that are likely to build up and cause side effects. It cannot predict every individual side effect.

Can I be tested if I am already taking an antidepressant?

Yes. Your results can help explain side effects or a poor response to your current medication and guide any changes. Keep taking your medication as prescribed while you wait for results.