It is already known that smoking can increase the risk of a number of cancers, including breast cancer. Now a new study has found that a treatment commonly used to treat women with breast cancer works less well in smokers.

Swedish researchers followed over 1,000 women who had been diagnosed with breast cancer between 2002 and 2012. One in five admitted to being regular or ‘social' smokers.

The study found that smokers being treated with aromatase inhibitors ‘had a three times higher risk of recurrence of breast cancer compared with the non-smokers who got the same treatment'.

"The study also showed that the smokers had an increased risk of dying either from the breast cancer or from other illnesses during the time we followed them," the researchers said.

Aromatase inhibitors are a class of drug used to treat breast cancer in postmenopausal women. They work by lowering levels of oestrogen in the body, which reduces the risk of recurrence in women with oestrogen-receptive positive breast cancer. Around 70% of cases of breast cancer are oesteogen-receptive positive.

"The treatment with aromatase inhibitors worked significantly better in the non-smoking patients. However, we saw little or no difference between smokers and non-smokers among patients treated with the drug tamoxifen, radiotherapy or chemotherapy. More studies are needed, but our findings are important as many breast cancer patients receive this type of treatment," the researchers from Lund University said.

They added that they were surprised that so few patients quit smoking during their treatment for breast cancer, despite being informed about how important this is.

Just 10% of the smokers stopped in the first year after their breast cancer surgery.

"That was unexpected. Smoking is not health promoting in any way, after all, so it is always beneficial to stop. But these findings show that patients who smoke need more support and encouragement to quit."

Details of these findings were published in the British Journal of Cancer.