Tamoxifen is a type of selective estrogen receptor modulator (SERM) used in breast cancer treatment. It’s also sometimes used to prevent breast cancer in those at high risk of the disease. The medication works by attaching to estrogen receptors within breast cells to minimize the effects of estrogen on breast tissue.

Though mostly taken by women, tamoxifen is also used by some men.

Read more: Breast cancer in men »

As with any medication, there’s always the risk of side effects. One concern with tamoxifen is the potential for weight gain. There’s little evidence to suggest that this is a common side effect, however. Many studies point to other causes of weight gain in people who take this medication.

Potential side effects of tamoxifen include:

  • blood clots
  • depression
  • hot flashes
  • menstrual cycle irregularities
  • spotting

Changes in weight are reported as a side effect among several medical agencies, but with conflicting reports. Some claim that weight gain is a potential side effect, while other sources indicate weight loss as a side effect.

Weight changes may not have one single cause. Weight gain can happen after breast cancer, especially in women. Tamoxifen may not necessarily be to blame.

Other possible causes of weight gain include:


Chemotherapy can cause more side effects than vomiting and hair loss. In fact, researchers have seen significant weight gain in women with breast cancer who’ve had chemotherapy. The reasons behind this link aren’t clear.

Hormonal changes from menopause

If you’re taking tamoxifen for menopause, there’s a chance that the weight gain might be from hormonal changes, rather than the medication.

Learn more: Weight gain and menopause »


Cancer and related treatments can significantly reduce your energy levels. This can cause reduced daily activities and exercise.

Dietary changes

Cancer treatments can impact your appetite, and even change the types of foods you crave. Gradual weight gain can happen as a result, especially if you start eating more carbohydrates, sweets, and processed foods.

Other undiagnosed health conditions

If you lead a balanced, healthy lifestyle, then there could be another underlying health issue that needs diagnosis, such as thyroid disease or diabetes.

Increased stress may also lead to weight gain.

Weight management can be tough after cancer. This is true whether you take medications like tamoxifen, or if your body is going through changes on its own.

Here are six ways you can help manage your weight after cancer:

1. Eat the right foods

Reducing the amount of insulin-triggering foods you eat can also help. When you eat brown rice instead of white rice, for example, the carbohydrates won’t cause blood sugar spikes and fat storage.

2. Don’t rely on counting calories only

When it comes to weight loss, as well as overall nutrition and well-being, fresh, whole foods should be emphasized over calories. A diet low in calories but high in simple carbs and processed foods is unlikely to yield results compared with a diet of fresh produce, lean meats, grains, and heart-healthy fats.

3. Track what you eat

You can track what you’re eating without counting calories. Chances are, you might be eating more than you realize, or eating more of a certain type of food, like take-out, than you thought. Keeping a log can help you monitor your eating habits and uncover opportunities for improvement.

4. Gradually start moving again

After cancer, you may not be able to hit the gym for a high-intensity workout. Instead of giving up on exercise altogether, gradually increase your activity level. Gardening, walking, and swimming are all good options. These types of activities can boost your mood too.

Read more: Tips for exercising with breast cancer »

5. Explore meditation

Deep breathing exercises can help manage stress hormones that contribute to weight gain. It can also help manage depression. Even a few minutes a day can make a difference on your outlook. If you’re ready to take meditation to another level, try integrative exercises such as tai chi and yoga.

6. Be patient

Finally, remember that weight loss can take time. It’s especially more challenging as you get older. If you still have difficulty managing your weight despite making lifestyle changes, talk to your doctor about possible medical interventions.

Weight gain is a possibility with hormonal therapies for cancer, but there isn’t enough evidence to concretely prove that this is inevitable with tamoxifen. Most people take tamoxifen for about five years. If you’re concerned about this medication as a source of your weight gain, talk to your doctor. You might consider switching to another type of SERM, if possible. You and your doctor will need to carefully consider the risks versus benefits of tamoxifen.