Medically reviewed by Dr. Douglas Blayney on November 9, 2023

We often hear the diagnosis of breast cancer and lose sight of the fact that there are several types of breast cancer. In the US, and much of the Western world, the most common form of breast cancer is called hormone-positive or estrogen receptor positive breast cancer. Hormone-positive breast cancer is a type of breast cancer that relies on hormones like estrogen and progesterone to signal the cancer cells to grow. Doctors can treat hormone positive with hormone therapies that help block the action of these hormones and slow down or stop the cancer from growing. Tamoxifen is one of these types of “hormone blocking” medicines for breast cancer that can lower the chance of it coming back and spreading. It is taken as a pill every day and works by protecting the cancer cells from the estrogen signaling growth signal. Tamoxifen clogs up the estrogen receptor in the cancer cell.  Like many medicines, Tamoxifen has side effects. One of the more annoying day-to-day side effects of manipulating estrogen and progesterone signaling in other parts of the body is weight gain. We will talk more about that below.

Tamoxifen and Breast Cancer

There are several subtypes of breast cancer, but the main ones are divided into three common categories (note that there can be overlap between the first two categories):

Hormone Receptor-Positive Breast Cancer: This type of breast cancer is sensitive to hormones like estrogen and progesterone. It includes subtypes like estrogen receptor-positive (ER+), progesterone receptor-positive (PR+), or both (ER/PR+).

HER2-Positive Breast Cancer: In this subtype, the cancer cells have too much of a protein called HER2. 

Triple-Negative Breast Cancer: This type does not have hormone receptors (ER/PR) or excess HER2 protein.

Tamoxifen is a drug used by some people who have been diagnosed with Hormone Receptor-Positive Breast Cancer. If we think of each cancer cell like a little car, hormones such as estrogen and progesterone signal l those cells and make it possible for them to spread. To stop those cells from getting the signal, doctors try to stop (or block) any estrogen or progesterone from being present in the body, or reaching the gas pedal. The theory is that if we take away the growth signal, we lower the risk of the cancer cells growing and spreading. That is why Tamoxifen is called a hormone-blocking medicine.

The link between Tamoxifen and weight gain

So why does taking Tamoxifen cause some people to gain weight? This is a question of particular interest to me, as I took Tamoxifen for 10 years and definitely experienced weight gain as one of the side effects of my treatment. Let’s first think about what the hormones estrogen and progesterone are meant to do for our body if we are healthy and unconcerned about cancer (wouldn’t that be nice?!).

What is estrogen?

Estrogen is a hormone that plays important roles in our bodies, especially in girls and women, though estrogen is present in men as well. Its main purpose is to help control and regulate things like the menstrual cycle and the development of female characteristics, such as breast growth, hair patterns, and a deeper voice. Estrogen also helps keep bones strong and healthy. It's like a natural messenger that tells the body what to do, and it's important for normal growth and functioning, particularly during puberty and in women's reproductive years. Here are some of the main roles of estrogen:

1. Regulating the Menstrual Cycle: Estrogen helps control the menstrual cycle in women, including the release of eggs from the ovaries.
2. Promoting Breast Development: During puberty, estrogen is responsible for the growth and development of breast tissue.
3. Maintaining Bone Health: Estrogen plays a role in keeping bones strong and healthy. It helps prevent bone loss and reduces the risk of osteoporosis.
4. Supporting Pregnancy: Estrogen levels rise during pregnancy and help maintain the uterine lining, necessary for a successful pregnancy.
5. Affecting Mood and Memory: Estrogen can influence mood and memory, and changes in estrogen levels are linked to mood swings and cognitive changes.
6. Skin Health: It contributes to skin health by maintaining skin thickness and promoting collagen production.
7. Vaginal Health: Estrogen helps maintain the health of the vaginal lining and can affect lubrication and elasticity.
8. Metabolism and Cholesterol: Estrogen has an impact on metabolism and can help control cholesterol levels in the blood.

What is progesterone?

Progesterone is another important hormone in our bodies, and its main purpose is to help prepare a woman's body for a possible pregnancy. It's like a helpful caretaker for the uterus. If a woman's egg gets fertilized by a sperm, progesterone helps make the uterus (womb) ready to support the growing baby. If there's no pregnancy, progesterone levels drop, leading to the start of the menstrual period. So, progesterone helps control the menstrual cycle and is a key player in pregnancy, making sure everything is ready for a baby if one is on the way. Here are some of the main roles of progesterone:

1. Preparing the Uterus: One of its primary roles is to prepare the lining of the uterus (womb) for a possible pregnancy. If an egg is fertilized by sperm, progesterone helps make the uterus a suitable place for the fertilized egg to implant and grow.
2. Regulating the Menstrual Cycle: Progesterone helps control the menstrual cycle by maintaining the uterine lining. If there is no pregnancy, a drop in progesterone levels triggers the start of the menstrual period.
3. Supporting Pregnancy: During pregnancy, progesterone levels remain high, maintaining the uterine lining and reducing the chances of miscarriage. It also helps relax the uterine muscles to prevent contractions.
4. Affecting Mood and Sleep: Progesterone can influence mood, making some women feel more relaxed or even drowsy. This hormone can also affect sleep patterns.
5. Breast Health: It plays a role in maintaining healthy breast tissue.
6. Cervical Mucus: Progesterone can affect cervical mucus, making it thicker and less receptive to sperm during non-fertile periods of the menstrual cycle.

So if we are blocking estrogen and/or progesterone, why might we gain weight?

One thing that estrogen does in the body is to impact metabolism. Metabolism is like the body's engine that uses energy from the food we eat to run everything, like breathing, moving, and even thinking. Some people have faster metabolisms, which means their bodies burn energy quickly, while others have slower metabolisms, which means their bodies use energy more slowly. There are many hormones in the body that impact how fast or slow your metabolism might be – including estrogen! When someone has hormone positive breast cancer, and the goal is to remove as much estrogen and/or progesterone from the cancer cell growth signaling as possible by using Tamoxifen to block the action of these hormones at the cancer cell.  Paradoxically, tamoxifen acts in some cells, such as the uterus, bone cells and fat cells like normal estrogen.  This is a good thing for reducing the amount of growth signal getting to cancer cells to help them grow, but tamoxifen, acting like estrogen on the other cells super-charges the estrogen effect  on the other cells. This can lead to undesirable side effects like weight gain, mood swings and depression, but also may stimulate bone growth.

Side effects of Tamoxifen

Some of the side effects of taking Tamoxifen might include:

• Menopause-like symptoms, including hot flashes, night sweats and vaginal dryness
• Weight gain (more common) or fluid retention (edema)
• Changes in menstrual periods for some women
• Tiredness or fatigue
• Changes in mood, like feeling sad or irritable
• Changes in how the uterus (womb) feels
• Hair thinning or hair loss
• Changes in the skin, like rashes or itching

Rarely, tamoxifen may cause serious issues such as:

• Blood clots, deep vein thrombosis (DVT) and strokes.
• Cataracts or other eye problems.
• Endometrial (uterine) cancer.

These side effects can vary from person to person, and it's important to talk to a doctor about any concerns or discomfort when taking Tamoxifen.

What to do about weight gain on Tamoxifen

I wish I had a tried and true solution to the dilemma of weight gain while using Tamoxifen. The truth is, there’s not one “silver bullet” answer to solve this. Here are some tips from our community for combatting weight gain… and don’t be annoyed that some of these are the old tried and true “diet and exercise” go-to’s. Trust me, it annoys me still because it’s really hard to deal with this on top of being worried about cancer – I get it! But regardless, here are those tips:

1. Maintain a Healthy Diet: Focus on a balanced diet rich in fruits, vegetables, lean proteins, and whole grains to help control your weight. But even if you “slip up” every now and then, do not beat yourself up – everything in moderation… even treats now and then.
2. Stay Active: Incorporate regular physical activity into your routine, such as walking, swimming, or other exercises you enjoy. Even a few minutes each day is better than nothing.
3. Strength Training: Include strength training exercises to help build muscle, which can boost metabolism.
4. Stay Hydrated: Drinking enough water can help you feel full and reduce the likelihood of overeating.
5. Limit Sugary and High-Fat Foods: Minimize the consumption of sugary snacks and high-fat foods, which can contribute to weight gain. I promise we’re not trying to take all the fun out of your life… just remember the ‘ol “everything in moderation” phrase!
6. Mindful Eating: Pay attention to hunger and fullness cues, and avoid emotional or stress eating. I found this to be difficult because… uh… cancer?! Can’t forget about that guy. And so be gentle with yourself if every now and then you partake in some emotional eating because… Cancer!
7. Regular Check-Ins: Keep regular appointments with your healthcare provider to monitor your health and discuss any concerns about weight changes.
8. Sleep Well: Aim for a consistent and adequate amount of sleep to support your metabolism and overall health.
9. Support System: Share your feelings with friends and family who can provide encouragement and help you process the complex life that comes with a cancer diagnosis. It’s also a good idea to consider talking to a mental health professional. You’re dealing with a lot and it’s ok to ask for help.

Remember, it's essential to consult your healthcare provider before making significant changes to your diet or exercise routine, especially while taking Tamoxifen. They can provide personalized guidance and recommendations based on your individual needs and health.

Other scientific studies about Tamoxifen

If you feel like getting into more details, here are some studies with additional data about Tamoxifen. You know, for light evening reading…

The Long and Short of Tamoxifen Therapy: A Review of the ATLAS Trial

ATLAS study looks at the benefit of taking Tamoxifen for 10 years instead of 5 years.

Assessment of 25-Year Survival of Women With Estrogen Receptor–Positive/ERBB2-Negative Breast Cancer Treated With and Without Tamoxifen Therapy

In this secondary analysis of data from 565 postmenopausal women with lymph node–negative, estrogen receptor–positive, and ERBB2-negative breast cancer who participated in the Stockholm tamoxifen randomized clinical trial (STO-3), tumor size and tumor grade were significantly associated with long-term (25-year) survival. A significant tamoxifen treatment benefit was observed among patients with larger tumors, lower tumor grades, and progesterone receptor–positive tumors.

Conclusion

We’ve discussed that there are different types of breast cancer, and this overview focuses on hormone-positive breast cancer, which depends on hormones like estrogen and progesterone to grow. To treat this type of cancer, medicines like Tamoxifen are used to block these hormones and slow down or stop cancer growth. However, one common side effect of Tamoxifen is weight gain, which is a very annoying part of the already crappy cancer experience. We hope you have a better understanding of what the purpose of Tamoxifen is in slowing down or preventing cancer cells, and also the side effects you might see when taking it. We, at Manta Cares, are walking this path with you. Join our community for more resources and information to help you navigate your own cancer experience. And know that you are not alone in your experience. We are here to help!

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