Lifestyle

Smoking means you inhale a number of cancer causing chemicals, like  polycyclic aromatic hydrocarbons, which are known to cause breast cancer.

Your risk of breast cancer from smoking is higher if you started smoking early and you have not reached the menopause.

Having children early (under 30) reduces your risk of breast cancer.Having more children, at a younger age, also reduces risk. Although the risk of breast cancer increases after having children, and peaks around 5 years after child-birth, the long-term protective effects of having children early outweigh this risk.

Breastfeeding your child gives you some protection against breast cancer, and the longer you do it the more you benefit – it is especially protective against developing hormone receptor negative breast cancer.

Most studies link combined oral contraceptives to a small increase in breast cancer risk. A recent Danish study of nearly two million women found a 20% increase in risk of breast cancer among current or recent users, compared to non-users. Increased risk was mainly associated with contraceptives containing combined synthetic oestrogen and progesterone.

Your risk goes up the longer you use the combined pill and decreases when you stop taking it, with risk no longer apparent five years after use has stopped. Intrauterine devices (IUDs), injections or other forms of combined hormonal contraception are also thought to increase risk.

It’s unclear whether hormonal contraceptives that contain progestin only (synthetic progesterone) increase risk, as results of studies vary.

There is an increased breast cancer risk whilst taking combined HRT (synthetic oestrogen and progesterone), and this increases the longer you take it. The risk lowers considerably when you stop taking it, although some excess risk persists for more than 10 years after stopping its use.  Most studies have found oestrogen-only HRT is associated with a small increase, or no, change in breast cancer risk, although this type of therapy may not be suitable for all women.

Night shift work and light at night exposure are thought to be risk factors for breast cancer, though not all studies support this finding.

Different assessments of what constitutes night shift work or different amounts of night light exposure may help explain the disparity in findings.

Night-time shift work and its correlation with increased breast cancer may be due to lower melatonin production (a hormone associated with circadian rhythm) which is associated with elevated breast cancer risk.

Low levels of vitamin D have been found in people with breast cancer.

It is unclear whether a low level of vitamin D increases risk, or whether breast cancer itself causes vitamin D levels to drop. Either way addressing this issue may be beneficial.