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“People respond to the pill in different ways” (Image: Natasha V/Masterfile/Corbis)

It’s one of the most efficient forms of contraception and has revolutionised reproductive control for women. One in four women of childbearing age in the UK and the US takes the contraceptive pill as a routine part of their daily schedule, often for reasons other than contraception (see graph).

The pill has drawbacks though. Last year, a review by the European Medicines Agency concluded that some of the bestselling combined contraceptive pills raise the risk of deep-vein thrombosis more than previously thought.

The packaging on these third-generation pills, so called because they contain new types of progestogen, has since been updated, and doctors were reminded to consider patients’ individual risk factors before issuing a prescription. These include being overweight, smoking and high blood pressure. The risk of blood clots is still small so, on balance, it is deemed to be outweighed by the benefits of preventing unplanned pregnancies.

In March, it was reported that the pill may raise the risk of Crohn’s disease, an inflammatory bowel condition, in women with a genetic susceptibility. And evidence also shows women on the pill have a higher risk of breast cancer.

There are hints that the pill might affect behaviour too, for instance, skewing what people find attractive in a partner. Perhaps ironically, some evidence shows that it can reduce libido.

The pill might also affect the way the brain functions. In April, a brain-scanning study found that two regions involved in emotion regulation, decision-making and reward response were , although the research gave no indication of whether this caused a real change in behaviour.

Confusingly, though, the pill has also recently gained attention for its health benefits. Data from 46,000 women observed for up to 39 years showed those who took the pill . Lead author at the University of Aberdeen, UK, thinks this is because the pill protects against some cancers.

Although it does raise a woman’s risk of breast cancer while she is taking it, Hannaford says most women take the pill during their 20s and early 30s, when the background risk is still low, so their chances of getting it are still very slim.

Other protective effects are longer lasting in women who take or have taken the pill. “They have a reduced risk of endometrial, ovarian and colorectal cancer and that effect seems to persist for many years after stopping – well into the age when those cancers become more common,” Hannaford says.

On balance, he says, the benefits outweigh the disadvantages, but women should make their choices based on contraception, rather than possible long-term health benefits.

What is becoming clear now, though, is that not everyone responds to the various contraceptive pills available in the same way. “The pill is certainly not for every woman,” says , at Sydney Medical School, who is studying its safety.

“People respond to the pill in different ways. It is certainly not for every woman”

One of the hardest areas to pick apart is the effect on mood. Many women anecdotally report mood swings or low mood, but the evidence is woolly at best. One recent analysis actually found pill users were than non-users.

, medical director of the Willow Women’s Clinic in Vancouver, Canada, says around 30 per cent of women using hormonal contraceptives will experience emotional and sexual side effects. But it’s hard to compare women on the pill with those who are not, because anyone who has experienced problems may just stop taking it without reporting this to their doctor, says Wiebe, which means the groups are self-selecting.

And often, she says, the studies are funded by the manufacturers themselves. They tend to look for symptoms of mental illness, such as suicidal thoughts, so subtler mood changes go unreported.

It’s also easy to assume that mood changes are down to relationship issues or life issues. “Women sometimes tell me that they’ve been on the pill since they were a teenager, and then went off it for some reason and discovered they were a different person. Only then did they realise they’d been having emotional side effects,” Wiebe says.

Read more: “Our daily pills: What everyday drugs are really doing to you“

This article appeared in print under the headline “The pill”