Tamoxifen (Nolvadex) is used to prevent and treat estrogen receptor-positive (ER+) breast cancer. Women who complete their prescribed anti-estrogen treatments have better recurrence and survival outcomes than those who do not. Some foods can enhance the effectiveness of tamoxifen treatment whereas others interfere with it.

Tamoxifen side effects

Since tamoxifen is life-saving, with proven effectiveness in preventing ER+ breast cancer and its recurrence, every effort should be made to complete treatment as prescribed. Side effects may be easier to tolerate if they are known to be typical. Knowledge of possible side effects can also encourage tamoxifen users to seek medical help, if needed.
The estrogen deprivation caused by tamoxifen results in some side effects (such as hot flashes) that are similar to symptoms experienced during menopause. Tamoxifen-induced estrogen deprivation can also cause some of the symptoms of chemo brain in some women. These symptoms appear to improve within a year after endocrine treatment ends (chemo brain may last longer if chemotherapy was part of breast cancer treatment). One major 2021 study found no links between use of endocrine treatments in non-metastatic breast cancer and risk of subsequent dementia.
Other side effects are specific to separate actions of the drug rather than estrogen suppression. Tamoxifen is associated with higher risk of certain gynecological side effects, including endometrial cancer in women who have not had a hysterectomy. However, there are some foods that can protect against endometrial cancer, again while not interfering with tamoxifen's effectiveness. Please see our article on the gynecological side effects of tamoxifen.
Tamoxifen is also associated with higher risk of developing blood clots. However, there are some foods that can protect against blood clots while not interfering with tamoxifen's effectiveness (see below). Tamoxifen can contribute to skin problems and hair loss in some women. There is also some evidence that long-term tamoxifen use might increase the risk of macular degeneration.

Tamoxifen influences cholesterol levels

Breast cancer typically eventually develops resistance to tamoxifen. Using tamoxifen can reduce cholesterol levels, which can rebound at the end of tamoxifen treatment. There is some evidence that breast cancer cells that don't respond to tamoxifen may be using cholesterol to shield themselves against the drug. While it is not clear that reducing dietary cholesterol would greatly affect this process, it may make sense for breast cancer patients on tamoxifen to reduce their cholesterol to low-normal levels.

Tamoxifen might increase risk of type 2 diabetes

It has been reported that tamoxifen use in breast cancer survivors is associated with an increased risk of type 2 diabetes. This risk may be higher in women who have other risk factors for diabetes.

Tamoxifen and body weight

There is some evidence that somewhat overweight women have a more favorable prognosis than normal weight women after tamoxifen treatment. However, one study reported that secretions from adipose stem cells of obese women were able to counteract the treatment effects of tamoxifen, suggesting that being overweight could contribute to tamoxifen resistance. In addition, weight gain, which is common during tamoxifen treatment, has been found to be associated with less favorable prognosis.

Micronutrients may influence tamoxifen treatment

Vitamin D has been shown to inhibit the growth of tamoxifen-resistant breast cancer cells in the laboratory, although not all studies are in agreement. Resveratrol has also been found to reduce their proliferation. Selenium and conjugated linoleic acid (CLA) may enhance the effectiveness of tamoxifen, whereas vitamin E, red clover and tangeretin have been shown to reduce it. Sulforaphane, a compound found in cruciferous vegetables, can cause a synergistic increase in cell death in combination with tamoxifen. Most of these micronutrients should be consumed as part of food rather than supplements (see Supplements, vitamins & other exposures below).
There is also some evidence that exposure to bisphenol A (BPA), parabens, or cigarette smoke can also interfere with the effectiveness of tamoxifen treatment.

Foods that enhance the effectiveness of tamoxifen treatment

The following foods (or major components) have been found to increase the anti-breast cancer effects of tamoxifen treatment or safely reduce its side effects:
1Not chicken livers or skin
Regular aspirin use appears to enhance the effectiveness of tamoxifen.

Foods that should be limited or avoided during tamoxifen treatment

The following foods (or major components) have been found to reduce the effectiveness of tamoxifen:
1See oranges for a more complete explanation.

Supplements for patients taking tamoxifen

There is no supplement or combination of supplements that has been proven to reliably reduce the risk of ER+ breast cancer recurrence in women. Attempting to take advantage of the apparent treatment effects of micronutrients and other dietary components by using supplements carries the risk of adverse and paradoxical effects, including promoting breast cancer growth and metastasis.
What is known is that when a beneficial micronutrient is administered at low doses by consuming food, it is likely to have subtle chemopreventive effects, whereas when the same micronutrient is administered at high doses, it is more likely to have pharmacological effects, with mostly unknown results. Therefore, it is best to obtain beneficial compounds by consuming food, if possible. Supplements should be used if medically necessary or to make up for deficiencies that are difficult to correct through diet.
The following supplements generally have been found to be safe and beneficial for those being treated with tamoxifen:
CoQ10 (if needed for heart health)100 to 400 mg/day
Fish oil (from wild-caught fish)1000 to 2000 mg/day
Vitamin D1000 to 2000 IU/day
Please consult your oncology team for advice concerning your situation and dosages. It might make sense to be tested for deficiency in vitamin D and plan for follow up to determine if your reading has reached a desirable level.

Supplements, vitamins & other exposures to be avoided on tamoxifen

The supplements below have been found to reduce the effectiveness of tamoxifen or increase its side effects. That is not to say that most other supplements are safe to take; there are no relevant scientific studies concerning the interactions between most supplements and tamoxifen.
There is abundant cell and animal study evidence that the curcumin found in turmeric can enhance the effectiveness of tamoxifen. Turmeric consumed as food is likely to be beneficial. However, turmeric or curcumin supplements could interfere with tamoxifen treatment (see Tamoxifen and turmeric or curcumin supplements).
Exposure to light at night suppresses melatonin production, which in turn can lead to tamoxifen resistance.

Foods that reduce the risk of blood clots

The foods below have been reported to decrease the likelihood of blood clots while protecting against breast cancer risk.

Foods that reduce cholesterol

The following foods have been shown to improve cholesterol profile while at the same time protecting against breast cancer:

Foods that increase cholesterol

The following foods have been shown to worsen cholesterol profile:

Additional comments

Tamoxifen is effective in preventing ER+ breast cancer and in treating early stage disease, but this effectiveness may be lost over time. Also, tamoxifen does not prevent ER- breast cancer. If the cancer progresses despite tamoxifen treatment, it is more likely to recur as ER- disease, which is normally more aggressive than ER+ breast cancer.
Therefore, it is important for women being treated with tamoxifen to eat a wide variety of the foods from our recommended list and limiting or avoiding those on our avoid list, in addition to paying particular attention to the foods listed above. Please see our article on how to optimize your breast cancer diet for information on what to eat during various stages of treatment and recovery.
For more information, please see the tamoxifen tag and Tamoxifen and alcohol.