Tamoxifen is a drug related to estrogen. It is called an agonist/antagonist, because it blocks the actions of estrogen and at the same time produces some estrogen-like actions. Tamoxifen is primarily used for the prevention and treatment of breast cancer.
Tangeretin is a bioflavonoid found in citrus fruit and some citrus bioflavonoid supplements. Animal studies suggest that high intake of tangeretin reduces the effectiveness of tamoxifen.1 For this reason, people using tamoxifen should avoid supplements containing tangeretin, and should also probably avoid excessive intake of citrus fruit.
Like tamoxifen, soy isoflavones have both estrogen-like and anti-estrogen actions. Test-tube and animal studies suggest that relatively low doses of soy isoflavones interfere with the ability of tamoxifen to inhibit breast cancer growth, but high doses of isoflavones augment the effectiveness of tamoxifen.1