Tamoxifen (Nolvadex) is effective in preventing and treating estrogen receptor-positive (ER+) breast cancer. Women who complete their prescribed anti-estrogen treatments have been found to have better recurrence and survival profiles than women who do not. There are some foods that appear to enhance the effectiveness of tamoxifen treatment and others that reduce it.
Tamoxifen is associated with higher risk of endometrial cancer for women who have not had a hysterectomy and higher risk of developing blood clots, among other side effects. However, there are some foods that can protect against endometrial cancer and blood clots while not interfering with tamoxifen's effectiveness.
Certain selective serotonin reuptake inhibitor (SSRI) antidepressants such as Paxil (paroxetine) and Prozac (fluoxetine), as well as the serotonin antagonist and reuptake inhibitor Oleptro (trazodone), have been found to interfere with tamoxifen treatment in some women. However not all studies have found a link. Exposure to light at night suppresses melatonin production, which in turn can lead to tamoxifen resistance. Regular aspirin use appears to enhance the effectiveness of tamoxifen. Preliminary evidence suggests that long-term tamoxifen use might increase the risk of macular degeneration.

Tamoxifen and body weight

There is some evidence that 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.

Tamoxifen and cholesterol

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.

Vitamin D and other micronutrients influence tamoxifen treatment

Vitamin D has been shown to inhibit the growth of tamoxifen-resistant breast cancer cells in the laboratory and resveratrol has been found to reduce their proliferation. Selenium and conjugated linoleic acid (CLA) may enhance the effectiveness of tamoxifen, whereas vitamin C, vitamin E, red clover and tangeretin have been shown to reduce it. In addition, brassica vegetables contain sulforaphane, an histone deacetylase (HDAC) inhibitor that may cause a synergistic increase in apoptosis and cell death in combination with tamoxifen. Most of these micronutrients should be consumed as part of food rather than supplements (see Supplements section below).
There is also some evidence that exposure to bisphenol A (BPA), parabens, or cigarette smoking could 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

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:
Alcohol
Clementine juice
Grapefruit & grapefruit juice
High protein diet
Orange or tangerine peel - found in orange tea, orange marmalade, Szechuan Orange dishes
Pineapple & pineapple juice
Red meat
Sesame seeds
Soy foods, especially soybeans, soy protein isolate, soybean paste
Vegetable oils with high omega-6: corn oil, safflower oil, soybean oil, sunflower oil

Supplements for patients taking tamoxifen

There is no supplement or combination of supplements that has been shown to reduce the risk of ER+ breast cancer recurrence. 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. The role of supplements is to make up for deficiencies that are difficult to correct through diet.
The following supplements generally been found to be safe and beneficial for those being treated with tamoxifen:
Supplement → Dosage
CoQ10 (only if needed for heart health) → 100 to 400 mg/day
Fish oil → 1000 to 2000 mg/day
Vitamin D → 1000 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. 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.
Apigenin supplements
Black cohosh supplements
Bromelain supplements
Danshen supplements
Goldenseal supplements
HRT, combined (estrogen plus progestin), including bioidentical hormones
L-carnitine supplements
Licorice root supplements
Milk thistle supplements
Quercetin supplements
Red clover supplements
Reishi mushroom supplements (ok for IBC)
Rhein supplements
Sage supplements & sage essential oil
Si-Wu-Tang supplements
Skullcap supplements
St. John’s wort supplements
Suan-Tsao-Ren-Tang supplements
Tangeretin supplements
Vitamin C supplements
Vitamin E supplements
There is abundant cell and animal study evidence that the curcumin found in turmeric can enhance the effectiveness of tamoxifen. However, curcumin supplements can interfere with tamoxifen treatment (see Tamoxifen and turmeric or curcumin supplements).

Foods that reduce the risk of endometrial cancer

The following foods (or major components) have been associated with lower risk of endometrial cancer while also protecting against breast cancer:

Foods that increase the risk of endometrial cancer

The foods and supplements listed below have been associated with increased risk of endometrial cancer. Sweets and other foods with high glycemic index, as well as animal fats, have been shown to increase the risk. Foods with high acrylamide content, such as French fries, cereal, potato chips, potatoes, and baked goods, have also been shown to increase risk of endometrial cancer. In addition, exposure to cadmium is associated with increased risk.
Long-term electric blanket use has also been found to be associated with increased risk of endometrial cancer and should be avoided.

Foods that reduce the risk of blood clots

The foods below have been reported to decrease the likelihood of blood clots while protecting against or being neutral with respect to 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 a selective estrogen receptor modulator (SERM), down-regulating ERα but not ERβ (in contrast, aromatase inhibitors such as Arimidex up-regulate ERβ). Tamoxifen is effective in preventing ER+ breast cancer and in treating early stages, but this effectiveness may be lost over time if the cancer progresses despite treatment. Also, tamoxifen does not prevent ER- breast cancer. Women who develop primary breast cancer or a recurrence despite tamoxifen treatment are at increased risk of developing ER- breast cancer, a more aggressive disease than ER+.
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 all stages of treatment and recovery.
For more information, please see tamoxifen or Tamoxifen and alcohol.