Basil (Ocimum basilicum) is an excellent source of vitamin K and a very good source of vitamin A (through its relatively high beta-carotene content). In addition, basil is a good source of lutein, rosmarinic acid, and ursolic acid, and is also a source of carnosol, estragole, eugenol and linalool. Basil is also known as sweet basil. It has antioxidant, anti-inflammatory, antifungal and neuroprotective properties.
Basil may help alleviate type 2 diabetes by improving insulin sensitivity. Carnosol, a component of basil, has been shown to inhibit carcinogenesis of human prostate and hormone receptor positive (ER+/PR+) breast cancer cells. While the evidence is mixed as to whether basil can retard liver tumor formation in experimental mice, it has been shown to inhibit liver cancer metastasis. Basil leaf extracts have been found to be highly effective in inhibiting carcinogen-induced lung tumor incidence in experimental mice, and to reduce the growth of glioblastoma and melanoma cells, as well as lung, bladder, colorectal, ovarian and pancreatic cancer cells.
Basil oil and its components have been shown to have significant antiproliferative activity in leukemia and kidney cancer cells. In addition, basil oil has been found to significantly inhibit carcinogen-induced squamous cell carcinoma in the stomachs of experimental mice. Purple basil has been reported to have a higher total phenolic acid content and greater antioxidant activity than green basil, based on its anthocyanin content (which gives the plant its purple color). This might translate into higher anti-cancer activity, although there is little available research on this topic.

Breast cancer-related effects of consuming basil

The recommendation for breast cancer patients and survivors to consume basil is based on cell and animal studies; there are few human studies. However, basil should be consumed in moderation and basil essential oil should be avoided, as explained below.

Cell studies

Basil extract has been shown in numerous studies to reduce proliferation and induce apoptosis (programmed cell death) in both ER+/PR+ and triple negative (ER-/PR-/HER2-) breast cancer cells.

Basil micronutrients

Beta-carotene
Women with substantial intake of carotenoids such as beta-carotene have been reported to have lower risks of breast cancer and its recurrence than those with low intake, although not all studies are in agreement.
A Scandinavian study found that dietary (but not supplemental) beta-carotene had a protective effect against lobular breast cancer in postmenopausal women. Another European study reported that high intake of beta-carotene was protective against breast cancer in postmenopausal women using hormone replacement therapy (HRT). The same study also found that dietary beta-carotene was associated with lowered risk of breast cancer in postmenopausal women with relatively high alcohol consumption.
Beta-carotene enhanced the cytotoxicity of Adriamycin (doxorubicin) in both ER+/PR+ and triple negative breast cancer cells in one study. Beta-carotene has also been demonstrated to reduce multidrug resistance in cancer cells.
Lutein
Basil is a good dietary source of the carotenoid lutein. Lutein consumption and circulating lutein levels have been found to be associated with reduced risk of breast cancer in several epidemiological studies. Lutein has been shown to inhibit the progression of both ER+/PR+ and triple negative breast cancer cells under hypoxia, a low-oxygen condition in which solid breast tumors can thrive. Lutein has also been shown to potentiate the effect of taxane chemotherapy drugs (Taxol and Taxotere) in breast cancer cells.
Rosmarinic acid
Although oregano and rosemary have far higher rosmarinic acid content, basil is also a source. Rosmarinic acid has been shown to reduce Adriamycin-induced cardiomyopathy (heart damage) without reducing its cytotoxic effects against ER+/PR+ breast cancer cells.
Ursolic acid
Basil is also a good source of ursolic acid, which has been shown to reverse multidrug resistance in breast cancer cells. One study found that ursolic acid reversed resistance to Taxol in Taxol-resistant triple negative breast cancer. Another study reported that ursolic acid resensitized multidrug resistant ER+/PR+ breast cancer cells to Adriamycin chemotherapy. Still another study reported that ursolic acid increased the sensitivity of triple negative breast cancer cells to Adriamycin.
Linalool
Linalool, another component of basil, has also been found to increase Adriamycin-induced cytotoxicity and pro-apoptotic effects in chemotherapy-resistant breast cancer cell lines.

Basil and breast cancer treatment

Basil consumption appears to be safe and beneficial during treatment with chemotherapy. As noted above, components of basil have been shown to reduce or reverse multidrug resistance, increase the cytotoxic effects of Adriamycin, Taxol and Taxotere in both ER+/PR+ and triple negative breast cancer cells, and protect against Adriamycin-induced heart damage.
Basil is not estrogenic and can be consumed during endocrine treatment for ER+ breast cancer. Carnosic acid, which is closely related to the carnosol found in basil, has been shown to increase the effectiveness of tamoxifen when combined with it in a mouse model of breast cancer..

Concentrated sources of basil are not recommended

Even organic basil tends to incorporate meaningful levels of heavy metals, which is one reason why it should be consumed in moderation.
When basil is used as a food ingredient, it is relatively safe, but basil essential oil might cause cancer in large quantities since it contains estragole. Estragole is an organic compound that acts as a rodent carcinogen in large doses. The proportion of estragole in basil essential oil can be significant.
Pesto sauce, which typically has basil as the primary ingredient, can also have a fairly significant estragole component. One Dutch study that performed a risk assessment of basil-based pesto sauces estimated that approximately 1 once per day could be consumed safely, based it's eugenol and estragole content. Pesto can also contain moderately high levels of copper, which could contribute to angiogenesis and metastasis of breast cancer, especialy in women with inflammatory breast cancer (IBC) or triple negative (ER-/PR-/HER2-) disease. Therefore, we recommend against consuming all but small amounts of basil essential oil and consuming basil pesto in moderation (perhaps two to four times per month).

Additional comments

While they are closely related, basil is not the same plant as holy basil (Ocimum sanctum), which is covered in its own webpage. Fresh or dried basil typically is used as a food ingredient, whereas holy basil normally is consumed as a herb in the U.S.
Non-organic basil must be washed very thoroughly to remove pesticide residue as much as possible.
Below are links to recent studies concerning this food and its components. For a more complete list of studies, please click on basil.