Left-sided breast cancer is more common than right-sided breast cancer, although researchers aren’t sure why.

According to a study published in the great journal Nature, breast cancer is more common in the left breast than in the right breast. The Surveillance, Epidemiology and Endpoints Program conducted an analysis of 881,320 people. The researchers found that left-sided tumors were observable in 50.8% of people and right-sided tumors were observable in 49.2% of individuals.

Although there are consistent reports that left-sided breast cancer is more common than right-sided breast cancer, research into the reasons for this is limited.
This article examines possible theories as to why left-sided breast cancer is more common than right-sided breast cancer. It also examines whether left or right breast cancer affects the outcome.

Why is breast cancer more common on the left side?

Experts still don’t know why left-sided breast cancer seems to be more common. Over the years, researchers have come up with various hypotheses to try to explain it, such as:

the larger size of the left breast
early detection of tumors in right-handed people
breastfeeding the right breast more often than the left breast
Both sides of the breast share the same genetic and environmental risk factors for the development of breast cancer. However, the two sides of the breast can have structural differences, including:

tissue structure
blood vessel supply
lymphatic drainage
This can create differences on each side of the breast, which may play a role in which side breast cancer occurs.

Other factors that research shows impact which side breast cancer occurs include:

the ratio of left-sided to right-sided breast cancer may vary with age
different areas of the breast may have different ratios depending on which side the cancer is growing on.
there may be a link between inherited genes and the side of the breast where the disease occurs.

Possible theories about left breast cancer

It is important to remember that none of the theories below are currently supported by enough research or scientific evidence.

Left breast size

In general, the left breast may be larger than the right. Some research suggests that there may be a link between breast size and breast cancer. However, a 2019 study found no clear evidence linking breast size directly to breast cancer risk.

Early detection due to being right-handed

According to a 2022 article, another theory is that right-handed people may detect signs of breast cancer in the left breast more easily than in the right, leading to earlier or more frequent diagnosis. Being right-handed is more common than being left-handed. About 90% of people choose to use their right hand to perform most tasks, and 10% prefer to use their left hand.

Breastfeeding right

Article 2022 also advances another theory that people who breastfeed feed more from the right side of the breast than the left side. Breastfeeding can reduce the risk of breast cancer. Researchers say that for every 12 months of breastfeeding, the risk of breast cancer decreases by 4.3%.

Does left-sided breast cancer have a poorer outlook?

Some research suggests that the side of the breast in which the cancer starts can affect the outlook for breast cancer. A large-scale analysis from 2022 compared the type of breast cancer tumors and outlook in people with left-sided and right-sided breast cancer.

The researchers found that the following features were more common in left-sided tumors than in right-sided ones:

poorly differentiated and undifferentiated tumors
hormone-negative and HER2-positive tumors
a higher number of grade 2 and 3 tumours.

After controlling for stage, grade, and hormone receptor status of breast cancer, left-sided breast cancer showed more negative outcomes in some study participants.
In other participants, research found no significant difference in outcomes.

The study concluded that left-sided tumors may have a poorer response to initial chemotherapy treatment and may have a slightly worse long-term outlook compared to right-sided tumors. However, researchers still need more evidence to support these findings.

Other research comes to different conclusions. A 2020 study of 228 people with breast cancer between 1999 and 2020 found that right-sided breast cancer had:

a higher rate of family history of breast cancer
more locally advanced and metastatic breast cancer
reduced 5-year survival rates depending on the size and stage of the breast cancer.
The study found that left-sided breast cancer is associated with early, higher-stage tumors.

The outlook for breast cancer can depend on many factors, including the stage, grade, and type of disease. They may also depend on the age of the person at the time of diagnosis and the recurrence of the cancer.


Breast cancer on the left side seems to be more common than that on the right side. However, the reasons for this phenomenon are unclear and research is limited. Some of the theories include that the left side of the breast is larger, that right-handed people check their left breast more easily, and that people prefer to breastfeed from the right breast. However, there is currently not enough evidence to support either of these theories. Some research suggests that the side where breast cancer occurs can affect the outlook. The size, grade and type of tumor are also important factors in the outlook for the disease.


Left sided breast cancer is associated with aggressive biology and worse outcomes than right sided breast cancer. Scientific Reports. 2022

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