Many people who take tamoxifen (Soltamox) worry about the side effects they might experience while on the medication. Common side effects include nausea, fatigue, hot flashes, rashes, and insomnia (trouble sleeping), which can be uncomfortable.

One side effect of tamoxifen some people are especially concerned about is unintentional weight gain. If you're noticing weight changes or are concerned about this possibility, here's a guide to tamoxifen, how it might relate to weight gain, and tips to help manage your health while on this medication.

What Is Tamoxifen?

Tamoxifen is a type of hormone therapy used to treat breast cancer that tests positive for hormone receptors. This means the cancer needs estrogen or progesterone to grow. Tamoxifen works by blocking estrogen from attaching to these receptors, which helps slow the growth of the cancer.

“I started tamoxifen last July and have gained 25 pounds. I walk five miles every day, and it hasn’t touched it.”

— A MyBCTeam member

Doctors often prescribe tamoxifen after chemotherapy or radiation to lower the chances of breast cancer returning (recurrence). Sometimes, it’s given to people over 35 years old who are at a high risk of developing breast cancer, which is often discovered through genetic testing. While tamoxifen is similar to drugs like anastrozole (Arimidex) and other aromatase inhibitors, it works differently by targeting estrogen receptors instead of lowering estrogen levels in the body.

Tamoxifen and Weight Gain: What the Research Says

Tamoxifen can cause side effects, and weight gain might be one of them for some people. However, research is mixed. Some studies suggest tamoxifen could impact weight by affecting thyroid function in the first year of treatment, which might make weight gain more likely. Other studies have found no strong connection between tamoxifen and weight gain.

It’s also important to know that weight gain may have other causes, especially after breast cancer treatment. For example, gaining 3 to 5 kilograms (about 6 to 11 pounds) is common after chemotherapy. Other factors, like changes in activity level, menopause, or recovery, can also play a role in weight changes.

MyBCTeam Members on Weight Gain and Tamoxifen

Still, some MyBCTeam members have connected weight gain they’ve experienced to their use of tamoxifen.

“After five months of taking it, I am noticing I suddenly have a growing upper belly that I never had before,” one member said. Another added, “I have gained 10 pounds since starting tamoxifen months ago.”

Additionally, tamoxifen can make it harder to lose weight or prevent weight gain. “I took tamoxifen from February 2008 to November 2014, and I did gain weight,” another member said. “I noticed that I couldn’t get rid of the belly fat.”

A fellow member replied, “I started tamoxifen last July and have gained 25 pounds. I walk five miles every day, and it hasn’t touched it. Oh, and mine is also in the belly, where I have never ever gained weight before.”

Such body weight changes can cause issues with body image and emotional distress. One member shared, “Tamoxifen also has me gaining weight in my tummy and my arms. I’m ashamed to wear sleeveless tops and dresses. I live in the South where it’s very hot.”

Whether your weight changes are related to tamoxifen, another breast cancer treatment, lifestyle factors, or another health condition, there are steps you can take to work toward a healthy weight. Focusing on your overall well-being can make a difference, and small changes in your routine may help you feel better physically and emotionally.

7 Tips for Managing Weight Gain While on Tamoxifen

There are a number of actions you can take to balance your weight while taking tamoxifen, or after you stop taking it. While it’s natural to be concerned about weight changes, it’s important to focus on supporting your overall physical health as your body goes through the changes of breast cancer treatment. Be gentle with yourself as you seek to balance your health, your weight, and your treatment plan.

1. Make Thoughtful Food and Drink Choices

Managing your weight doesn’t mean depriving yourself. You can still nourish your body while making small substitutions that have a big impact. Choose beverages with less sugar, and replace junk food with healthier options like whole-grain snacks or fresh fruit. These changes can help you feel satisfied and may make a difference as you work to manage your weight during or after breast cancer treatment with tamoxifen.

2. Track Your Eating Habits

Tracking what you eat can help you understand your eating habits and identify where small changes can make a difference. For example, you might notice you’re eating more than you realized or choosing less healthy options. Use a food-tracking app, jot meals in a notebook, or take photos of your food. This awareness can help you adjust your portions or food choices to align with your goals. If you’re unsure where to start, a dietitian can help create a personalized plan.

3. Eat More Fiber-Rich Foods

If you’re often tempted to overeat, adding more fiber to your diet may help. Fiber-rich foods like fruits, vegetables, whole grains, and legumes (such as beans, lentils, and peas) help you feel fuller for longer. By making fiber a bigger part of your meals, you might not need to eat as much to feel satisfied. This can make managing portions easier while still enjoying nourishing foods that support your health.

4. Limit How Much Alcohol You Drink

Alcoholic drinks are often high in calories, and these can add up quickly, especially if you drink often. Cutting back on alcohol or eliminating it altogether can help you manage your weight. For example, choosing water, sparkling water, or other low-calorie nonalcoholic drinks can be a good substitute.

Weight gain may also be caused or worsened by menopause, fatigue due to treatment, or recently quitting smoking.

If you choose to drink alcohol, opting for a plain spirit (like vodka or gin) with a no-calorie mixer, such as soda water, can help keep calories lower. However, it’s worth noting that alcohol may also increase the risk of breast cancer recurrence, though research is mixed. It’s a good idea to talk with your doctor about your alcohol consumption.

5. Learn To Balance Insulin

Understanding insulin resistance can help you manage weight changes. Insulin is a hormone that controls your blood glucose (sugar) levels. When your body becomes resistant to insulin, it doesn’t use it as effectively, which can affect your metabolism and possibly lead to weight gain. One member of MyBCTeam shared, “I began to understand that insulin resistance might be the key.”

Your doctor can check your insulin levels and help you determine if insulin resistance is a factor for you. Even if your insulin levels are stable, eating in a way that keeps your blood sugar steady may still be helpful. Focus on fresh, whole foods, lean proteins, and high-fiber options while avoiding sugary foods, white bread, and processed carbs. These types of foods can cause blood sugar spikes that may make it harder to manage your weight.

6. Stay Physically Active

After breast cancer treatment, it’s understandable if you haven’t been able to exercise as much as you’d like. However, adding movement to your daily routine can be a great way to manage your weight and improve your overall health.

You can start small — even short walks or gentle stretches can make a difference. Over time, you can gradually build a routine that fits your energy levels and feels enjoyable, whether it’s yoga, swimming, or light strength training. Regular physical activity can also boost your mood, reduce fatigue, and help you feel stronger overall.

7. Consider Other Causes for Weight Gain

You may want to consider whether your weight changes have other causes. A wide variety of medications can cause weight gain, and many of them are not related to breast cancer. Hormonal changes unrelated to breast cancer or tamoxifen, including menopause, can also play a role.

Fatigue after treatment may make it harder to stay active, while weight gain could also result from recovery as your body becomes healthier. Even quitting smoking can lead to weight changes. Identifying the cause can help you find effective ways to address it with your doctor’s support.

Talk to Your Doctor

If you’re gaining weight and you aren’t sure why, talk to a healthcare professional or your oncology team. Your oncologist may be able to run tests to determine what is causing your weight gain, which may lead to more effective ways of managing it. Additionally, they may be able to direct you to dietitians who can provide you with a plan for healthy eating.

Talk With Others Who Understand

On MyBCTeam — the social network for people with breast cancer, and their loved ones — more than 76,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Have you experienced weight changes while taking tamoxifen? Do you have advice for other breast cancer survivors? Share your experience in the comments below, start a conversation by posting on your Activities page, or connect with like-minded members in Groups.