Everyone knows smoking causes lung cancer. Fewer people know it's also linked to breast cancer. According to the Breast Cancer Research Foundation, smokers have about a 16% higher risk of developing breast cancer than women who have never smoked. In comparison, former smokers have about a 9% higher risk.
Why age of starting matters
Breast tissue continues to develop through adolescence, making it more sensitive during those years than it will be later in life. The riskiest window is before a first period settles into a regular cycle, and before a first full-term pregnancy, both markers of breast tissue still forming.
Carcinogens in tobacco smoke don't stay in the lungs. They circulate through the bloodstream and reach breast tissue, where there's decent evidence they can damage DNA.
What about secondhand smoke?
Living for years with a smoking partner, or growing up in a smoking household, shows up in some studies as a modest risk increase in people who never smoked. It's a smaller effect than direct smoking, but it isn't something to neglect.
Other risk factors
Alcohol consumption has a more consistently documented effect on breast cancer risk across studies, even at moderate drinking levels. Obesity, particularly after menopause, and long-term use of certain hormone therapies also carry stronger, more uniform associations than smoking does.
What this doesn't mean
Not every smoker gets breast cancer, and not every case in a smoker was "caused" by smoking. Quitting doesn't erase risk instantly either; it drops gradually over time, not back to zero overnight. Breast cancer risk depends on many factors, including genetics, family history, reproductive history, and alcohol use. That’s why it’s rarely possible to say smoking alone caused one person’s cancer.