Smoking increases the risk of breast cancer, but only before menopause, study finds

Among the many harmful substances found in tobacco smoke are some that are thought to cause breast cancer. Among those, researchers have flagged polycyclic aromatic hydrocarbons, aromatic amines and N-nitrosamines.

Many studies have investigated the link between smoking and breast cancer, and their conclusions have been mixed. On Monday, a group of Harvard researchers used 30 years worth of data from the long-running Nurses Health Study to conclude that smoking does lead to a modest increase in breast cancer risk, but only in premenopausal women.

The data included 111,140 women; among them, there were 8,772 cases of invasive breast cancer. Altogether, the researchers concluded that smokers and former smokers had a 6% to 7% increased risk of breast cancer. The risk was higher for women who started smoking before age 17, smoked at least 25 cigarettes per day at any point in their lives, and smoked for at least 20 years. Worst-off were women who picked up the habit before age 18, smoked for at least 36 years and smoked at least 26 cigarettes per day – for them, the risk of breast cancer was 25% higher.

However, once women hit menopause, smoking was linked to a slight reduction in the risk of breast cancer. The link wasn’t statistically significant, but it didn’t come from out of the blue. Researchers have suspected that smoking has an anti-estrogenic effect based on epidemiological studies linking the habit to an earlier onset of menopause, a reduced effect of hormone therapy and an increased risk of osteoporosis.

The researchers also found that exposure to second-hand smoke had no effect on breast cancer risk.

The study was published in Archives of Internal Medicine.

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