Quercetin, naringenin, naringin | These three flavonoids in citrus fruits are anti-estrogens
Citrus peel contains the flavonoids quercetin, naringenin and naringin. All these three substances have an anti-oestrogenic effect. A multiple anti-oestrogenic effect, we should actually say.

Egyptian pharmacologists tell in Scientific Reports that they injected mice with an estradiol-sensitive type of cancer cell. After the cells had grown into tumors, the researchers injected a daily dose of quercetin, naringin or naringenin into the mice's small intestines for 7 days.

In other experiments, which they also described in this article, the Egyptians had determined that quercetin, naringin and naringenin were the three strongest aromatase inhibitors they could find in citrus.
If the mice had been humans who had taken the flavonoids orally, they would have taken about 60 to 100 milligrams of quercetin, naringin or naringenin daily. This is a very rough estimate, by the way, but we don't have anything better.
Supplementation with quercetin, naringin and naringenin inhibited the growth of the tumors and reduced the concentration of aromatase in the tumors. You can see this below.

Using in silico models, the researchers discovered that quercetin, naringin and naringenin can have an anti-oestrogenic effect because their spatial structure resembles that of estradiol. The enzyme mistakenly recognizes them as a testosterone or androstenedione analogue, so the flavonoid molecules partly displace testosterone and its analogues from the enzyme. This reduces the production of estradiol

At the same time, quercetin, naringin and naringenin are so similar to estradiol that they displace some of this hormone from its receptor. This mechanism reduces the number of estradiol molecules that are actually active.
"This study suggests that flavonoids isolated from citrus peels exhibit anti-estrogenic and anti-aromatase activities suggesting their potential prophylactic and therapeutic use in both pre- and postmenopausal breast cancer patients," summarize the Egyptians.

Sci Rep. 2021 Mar 29;11(1):7121.