Tamoxifen has been prescribed for decades to help reduce the risk of breast cancer recurrence in women with estrogen receptor-positive (ER+) breast cancer. For many women, it is an important part of their treatment plan and we are told that it has saved countless lives.

Like all medications, however, tamoxifen can cause side effects. One that isn’t always talked about enough is its impact on mood.

As a breast cancer coach, I’ve spoken with many women over the years who have described feeling unlike themselves after starting tamoxifen. Some report persistent sadness, low motivation, anxiety, emotional numbness, or what they describe as “living under a dark cloud.” One woman told me she felt absolutely suicidal while taking it. Others experience very few emotional side effects at all. One thing is for sure – every woman’s experience is different.

Depression and anxiety after a breast cancer diagnosis is also common for many reasons. The diagnosis itself can be emotionally overwhelming, and treatment often brings physical exhaustion, disrupted sleep, hormonal changes, uncertainty about the future, and significant life adjustments. Studies suggest that approximately one in four women with breast cancer experience clinically significant depression during their cancer journey. My article “11 Anxiety Busters for Breast Cancer Patients” might be helpful to you if you are suffering from anxiety.

Depression is something else, though, and recognizing and treating depression is important. Depression affects quality of life, relationships, recovery, and even adherence to cancer treatments. No woman should feel she simply has to “push through” without support.

One common approach is for a physician to prescribe an antidepressant. In many situations, this may be entirely appropriate. However, if you’re taking tamoxifen, it’s important to know that not all antidepressants are the same.

Some antidepressants can interfere with the way tamoxifen is processed in the body, potentially reducing the amount of its active cancer-fighting metabolite. This doesn’t necessarily mean they should never be used, but it does mean that choosing the right antidepressant matters.

Fortunately, doctors today understand this interaction much better than they did a decade ago, and there are often suitable alternatives available.

Understanding How Tamoxifen Works

Tamoxifen belongs to a group of medications known as Selective Estrogen Receptor Modulators (SERMs). Although it’s often described as an “estrogen blocker,” tamoxifen’s actions are actually much more complex than that. Depending on the tissue, it can either block or mimic some of estrogen’s effects.

In breast tissue, tamoxifen blocks estrogen receptors, helping to prevent estrogen from stimulating the growth of hormone receptor-positive breast cancer cells. In other parts of the body, however – including bone, the uterus, and the brain – its effects are more complex.

Estrogen itself plays many important roles beyond reproduction. It helps support bone health, cardiovascular health, cognitive function, vision, and influences neurotransmitters involved in mood regulation, including serotonin and dopamine. Researchers continue to investigate how changes in estrogen signaling may contribute to mood symptoms experienced by some women during endocrine therapy. While the exact relationship isn’t fully understood, it’s likely that mood changes result from a combination of hormonal effects, the stress of cancer treatment, sleep disruption, and each individual’s unique biology.

This helps explain why some women notice emotional changes while taking tamoxifen, whereas others tolerate the medication remarkably well.

Why Some Antidepressants Can Interfere with Tamoxifen

Here’s where things become especially important. Tamoxifen itself isn’t actually the most active form of the drug. After you swallow a tamoxifen tablet, your liver converts it into several metabolites. The most important of these is called endoxifen, which is responsible for much of tamoxifen’s anti-cancer activity. This conversion depends largely on an enzyme called CYP2D6.

Some antidepressants – particularly certain Selective Serotonin Reuptake Inhibitors (SSRIs) – can partially block this enzyme. When CYP2D6 is strongly inhibited, the body may produce lower levels of endoxifen, potentially reducing tamoxifen’s effectiveness.

This interaction has been recognized for many years and is now well established pharmacologically. What researchers have continued to investigate is whether this reduction in endoxifen actually translates into higher rates of breast cancer recurrence or mortality. While studies have produced mixed results regarding long-term clinical outcomes, most experts agree that strong CYP2D6 inhibitors should be avoided when reasonable alternatives are available, particularly because several antidepressants have little or no effect on this enzyme.

That doesn’t mean women should avoid treatment for depression. Untreated depression carries its own significant health consequences and deserves proper attention.

Rather, it means that if you’re taking tamoxifen and need an antidepressant, it’s worth having a conversation with your doctor or pharmacist about choosing one that’s less likely to interfere with tamoxifen’s metabolism.

If you’re suffering some significant side effects with tamoxifen, you might find this article helpful: Coping with Tamoxifen Side Effects

Your Genes May Also Play a Role

Another interesting piece of the puzzle is that not everyone processes tamoxifen in exactly the same way. The CYP2D6 enzyme is produced according to instructions in the CYP2D6 gene, and we all inherit slightly different versions of this gene from our parents. As a result, some people naturally convert tamoxifen into its active form, endoxifen, more efficiently than others.

Researchers generally classify people into four metabolizer groups:

1. Poor metabolizers, who produce very little CYP2D6 enzyme.

2. Intermediate metabolizers, who produce less than average.

3. Normal (or extensive) metabolizers, who process tamoxifen as expected.

4. Ultrarapid metabolizers, who convert tamoxifen more quickly than most people.

Women who are poor metabolizers may naturally have lower endoxifen levels, even if they aren’t taking any medications that interfere with CYP2D6. Conversely, someone who normally metabolizes tamoxifen well could experience lower endoxifen levels if they begin taking a strong CYP2D6-inhibiting antidepressant.

This is one reason why the interaction between tamoxifen and antidepressants isn’t always straightforward. Each woman’s genetics, medications, and overall health all contribute to how effectively tamoxifen works in her body.

Researchers continue to investigate whether CYP2D6 genetic testing should play a larger role in personalizing tamoxifen therapy. At present, however, most clinical guidelines do not recommend routine genetic testing for everyone taking tamoxifen. Instead, healthcare providers generally focus on avoiding medications known to strongly inhibit CYP2D6 whenever practical.

Keep in mind that genes are only one piece of the picture. Our genetics influence how we respond to medications, but factors such as age, liver function, other medications, adherence, and overall health also matter.

What the Research Has Found

The question of whether antidepressants can interfere with tamoxifen first gained widespread attention in 2010, when Canadian researchers published a study that sparked considerable discussion among oncologists, pharmacists, and breast cancer specialists.

The Study That Raised the Alarm

Researchers in Ontario, Canada, followed 2,430 women aged 66 years and older who were treated with tamoxifen between 1993 and 2005. They investigated whether taking SSRI drugs at the same time as tamoxifen affected breast cancer outcomes. Their findings were striking.

Women who took paroxetine (Paxil) while receiving tamoxifen had a significantly higher risk of dying from breast cancer compared with women who did not take paroxetine. The longer the two medications were taken together, the greater the observed risk. The researchers estimated that if paroxetine overlapped with tamoxifen treatment for approximately 40% of the treatment period, it could result in one additional breast cancer death within five years for every 20 women treated in this way.

Importantly, the increased risk was not seen with every antidepressant studied. The strongest association was found with paroxetine, one of the most potent inhibitors of the CYP2D6 enzyme.

This study was among the first to suggest that reducing the body’s ability to convert tamoxifen into endoxifen might have meaningful clinical consequences.

Later Studies Painted a More Complex Picture

As often happens in medical research, one important study led to many more. Several research groups attempted to determine whether the Canadian findings would hold true in larger and more diverse populations.

One of the largest studies, reported in 2015, followed 16,887 breast cancer survivors receiving tamoxifen over a period of up to 14 years. Approximately half of these women also used antidepressants, including paroxetine.

Unlike the Canadian study, these researchers did not find an increased risk of subsequent breast cancer or breast cancer mortality associated with concurrent antidepressant use.

At first glance, these results appeared contradictory. However, researchers have pointed out that differences in study design, patient populations, treatment duration, medication adherence, genetics, and follow-up periods can all influence the findings of observational studies. Rather than proving one study was “right” and the other “wrong,” the differing results highlighted just how complex real-world medication research can be.

What We Know Today

Although researchers continue to study this question, one aspect is no longer in doubt – certain antidepressants do reduce CYP2D6 activity. Because CYP2D6 is responsible for converting tamoxifen into its most active metabolite – endoxifen – strong inhibitors of this enzyme can produce substantially lower endoxifen levels in the bloodstream.

This pharmacological interaction has been demonstrated repeatedly and is widely accepted. The remaining question has been whether those lower endoxifen levels consistently translate into poorer long-term cancer outcomes. The answer, according to current evidence, appears to be:

Not always – but it’s still considered sensible to avoid the strongest CYP2D6 inhibitors whenever suitable alternatives are available.

What Do Experts Currently Recommend?

Over the past several years, reviews of the available evidence have reached a fairly consistent conclusion. Most experts recommend that women taking tamoxifen should avoid antidepressants that strongly inhibit CYP2D6 if an equally effective alternative is available, and fortunately, many alternatives do exist.

Antidepressants such as venlafaxine (Effexor), desvenlafaxine (Pristiq), escitalopram (Lexapro) and citalopram (Celexa) have much less effect on CYP2D6 and are often preferred when antidepressant treatment is needed during tamoxifen therapy.

In contrast, paroxetine (Paxil) and fluoxetine (Prozac) are generally considered strong CYP2D6 inhibitors and are usually avoided whenever possible in women taking tamoxifen.

That doesn’t necessarily mean they can never be prescribed. Every patient’s situation is unique, and for some women, the benefits of remaining on a particular antidepressant may outweigh the theoretical risks of changing medications. These decisions should always be made together with the treating physician.

Current pharmacy and oncology guidelines increasingly emphasize medication reviews to identify potential CYP2D6 interactions before treatment begins.

What If You’re Taking an Aromatase Inhibitor Instead?

You may be wondering whether this information also applies if you’re taking an aromatase inhibitor (AI) such as anastrozole (Arimidex), letrozole (Femara) or exemestane (Aromasin).

The answer is not in the same way.

Unlike tamoxifen, aromatase inhibitors do not rely on the CYP2D6 enzyme to become active. Instead, they work by reducing the body’s production of estrogen by inhibiting the enzyme known as aromatase. Because of this, the well-known CYP2D6 interaction between tamoxifen and certain antidepressants does not apply to aromatase inhibitors.

That said, depression, anxiety, mood changes and cognitive symptoms can still occur while taking aromatase inhibitors. As mentioned before, estrogen influences many aspects of brain function, including mood, memory and emotional regulation. For some women, the profound reduction in estrogen caused by aromatase inhibitors may contribute to these symptoms, although factors such as the emotional impact of cancer, sleep disturbance, pain and fatigue also play important roles.

If you’re taking an aromatase inhibitor and are experiencing symptoms of depression, it’s important to discuss them with your healthcare team. While the concerns about CYP2D6 interactions don’t apply, your doctor or pharmacist should still review your medications for other potential drug interactions and help you choose the treatment that’s most appropriate for you.

The Importance of Treating Depression

One point deserves special emphasis. Sometimes women become understandably worried after reading about potential drug interactions and consider stopping their antidepressant medication.

Please don’t make changes without first speaking with your doctor.

Depression is a serious medical condition that deserves appropriate treatment. Untreated depression can affect sleep, relationships, energy levels, motivation, treatment adherence, and overall quality of life. In some cases, it may even interfere with a woman’s ability to continue life-saving cancer therapy.

The goal isn’t to avoid antidepressants altogether – the goal is to ensure that, when they’re needed, the safest and most appropriate antidepressant is chosen for someone taking tamoxifen.

Looking After Your Emotional Wellbeing During Tamoxifen Treatment

If you’re taking tamoxifen and struggling with depression, anxiety, or low mood, the first thing I’d like you to know is this: 

You’re not alone, and you don’t have to suffer in silence.

For many women, breast cancer is one of the most emotionally challenging experiences of their lives. Even after active treatment has finished, there can be lingering fears about recurrence, changes in body image, disrupted sleep, relationship challenges, financial pressures, and the ongoing adjustment to a “new normal.”

On top of this, some women find that endocrine therapies such as tamoxifen affect their emotional wellbeing, while others notice very few mood changes at all. Every woman’s journey is unique.

The important thing is to recognize when you’re struggling and to ask for help. Depression isn’t a sign of weakness, nor is it something you should simply “push through.” Effective support is available, and reaching out is one of the strongest things you can do for yourself.

Questions to Ask If You’re Prescribed an Antidepressant While On Tamoxifen

If you’re taking tamoxifen and your doctor recommends an antidepressant, it may be helpful to ask a few questions before starting treatment. For example: 

• Does this antidepressant affect the CYP2D6 enzyme?
• Is there another medication that may work just as well with less potential to interfere with tamoxifen?
• Would it be appropriate for my pharmacist to review my medications for possible interactions?
• What side effects should I watch for?
• How will we know whether this medication is helping?

These questions should not be challenging to your healthcare team – they’re simply part of being an informed participant in your own care. Most doctors appreciate patients who take an active interest in understanding their treatment options.

Supporting Emotional Health Naturally

Over the past years of working with women affected by breast cancer, I’ve learned that emotional healing rarely comes from one single solution. Medication can be incredibly helpful for some women. Counseling may be life-changing for others. Often, it’s a combination of approaches that brings the greatest improvement.

Many women also find that healthy lifestyle habits provide meaningful support for emotional wellbeing. While these approaches shouldn’t replace appropriate medical care when it’s needed, they can become valuable parts of an overall healing plan.

Some of the strategies I regularly encourage include:

•  Eating a nourishing, anti-inflammatory diet rich in colorful vegetables, fruit, quality protein, healthy fats, herbs and spices (I’ve got a cookbook to help you with this).
• Gentle daily movement, even if it’s only a short walk outdoors.
• Prioritizing a good night’s sleep wherever possible.
• Spending time in nature, which has been shown to reduce stress and improve mood.
• Practicing meditation, mindfulness, breathwork, or prayer.
• Connecting with supportive family members, friends, or breast cancer support groups.
• Seeking professional counseling when emotions become overwhelming.

Research consistently shows that regular physical activity, nutritious eating patterns, good sleep, stress-management practices, and strong social support can improve quality of life and emotional wellbeing during and after breast cancer treatment. While these approaches may not replace treatment for clinical depression, they can be valuable components of a comprehensive care plan.

My Personal Perspective

As many of my readers know, I have always been passionate about natural approaches to health and healing.

I believe our bodies have an extraordinary capacity for healing when we provide them with the nutrients, movement, rest, emotional support, and healthy lifestyle habits they need.

Throughout my years as a breast cancer coach, I’ve seen women benefit from a wide range of complementary approaches, including good nutrition, appropriate supplementation, stress reduction techniques, mindfulness, exercise, essential oils, herbal medicine, and other supportive therapies.

These approaches aren’t intended to replace medical treatment. Rather, they can work alongside conventional care to help support the whole person – body, mind, and spirit.

Healing is about much more than simply treating a disease – it’s about helping a woman feel like herself again.

The Bottom Line

If there’s one message I’d like you to take away from this article, it’s this:

If you’re taking tamoxifen and need treatment for depression, don’t be afraid to seek help. Depression deserves attention and treatment. At the same time, it’s worth making sure that any antidepressant prescribed is compatible with tamoxifen, if you’re taking it.

Today, healthcare providers understand this interaction far better than they did years ago, and there are several antidepressants available that have little effect on the CYP2D6 enzyme responsible for activating tamoxifen.

A simple conversation with your oncologist, doctor, or pharmacist may be all that’s needed to ensure you’re receiving the most appropriate treatment for both your emotional well-being and your long-term breast cancer care.

Knowledge truly is empowering. The more you understand your treatment options, the better equipped you’ll be to work together with your healthcare team and make informed decisions that support both your physical and emotional health.

Every treatment decision comes with questions, and my goal is to help you understand the evidence so you can make informed choices with confidence.

 

If this article was helpful, you may also enjoy these articles on Substack (another helpful resource where I post more regularly):

• From Anxiety to Clarity: Coping with Fear After a Breast Cancer Diagnosis

• When Breast Cancer Treatment Leaves You Dry “Down There” – Natural ways to ease vaginal dryness and improve comfort during endocrine therapy.

• How Long After Chemo Will I Feel Better? – Understanding recovery after chemotherapy and what you can do to support healing.

• Healing Beyond the Physical: Why Spirituality Matters on the Healing Journey – Looking after your emotional and spiritual wellbeing alongside your physical recovery.

References:

Article: American Society of Clinical Oncology (ASCO): Exercise, Diet, Weight Management During Cancer Treatment – https://www.cancer.net/survivorship/healthy-living

2025 Article: Clinical Update 2024: Recommendations on the Use of Antidepressants in Women Taking Tamoxifen – https://womensmentalhealth.org/posts/clinical-update-tamoxifen-and-antidepressants/

2010: Breast Cancer Recurrence Risk Related to Concurrent Use of SSRI Antidepressants and Tamoxifen – https://pmc.ncbi.nlm.nih.gov/articles/PMC2892037/

2010: Selective Serotonin Reuptake Inhibitors and Breast Cancer Mortality in Women Receiving Tamoxifen: A Population Based Cohort Study – https://pmc.ncbi.nlm.nih.gov/articles/PMC2817754/

2010: Coprescription of Tamoxifen and Medications That Inhibit CYP2D6 – https://pmc.ncbi.nlm.nih.gov/articles/PMC2881853/

2015: Tamoxifen and Antidepressant Drug Interaction in a Cohort of 16,887 Breast Cancer Survivors – https://pubmed.ncbi.nlm.nih.gov/26631176/

2018: Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2D6 and Tamoxifen Therapy – https://pmc.ncbi.nlm.nih.gov/articles/PMC5931215/