At a time when women’s reproductive freedoms are under attack, any suggestion that the birth control pill could be problematic feels explosive. But Sarah E Hill, a professor of social psychology at the Texas Christian University in Fort Worth, Texas argues we need to talk about how oral contraceptives are affecting women’s thinking, emotions and behaviour. How the Pill Changes Everything: Your Brain on Birth Control is her new book about the science behind a delicate subject.

Some US states have recently made it harder to get an abortion and the Trump administration is doing its best to chisel away at access to birth control. Is your book trying to dissuade women from using the pill?

My institution was founded as a Christian school, but it doesn’t have a particular religious bent now. My goal with this book is not to take the pill away or alarm women. It is to give them information they haven’t had up until now so they can make informed decisions. The pill, along with safe, legalised abortions, are the two biggest keys to women’s rights. But we also have a blind spot when it comes to thinking about how changing women’s sex hormones – which is what the pill does – influences their brains. For a long time, women have been experiencing “psychological” side-effects on the pill but nobody was telling them why.

The backlash we are seeing against the pill, particularly with millennial women walking away from it, I think is because women haven’t felt right on it and have grown weary of doctors patting them on their heads and telling them they are wrong. The more information women have, the more it will bring them back to the pill.

How exactly does the pill affect women’s brains?

The pill works mainly by mimicking the second half of a woman’s monthly ovulatory cycle, in which the hormone progesterone is dominant. Its primary ingredient is artificial progesterone – so-called progestin – which shuts down the brain signal that prompts egg development. But that also means the ovaries aren’t producing oestrogen, the hormone which dominates the first half of a woman’s natural cycle. And oestrogen does some of our favourite things to our brains and bodies, making us feel sexier and more energetic. While most pills include some synthetic oestrogen – primarily to offset progestin’s unpleasant side-effects – women can feel the lack.

A less well-understood mechanism is that progestins, which are often created by changing the molecular structure of testosterone, aren’t an exact match for our progesterone receptors. This means they can potentially stimulate other receptors, including those for testosterone, leading to masculinising effects, and for cortisol, which over time can dysregulate stress response. The stress hormone profile of pill-taking women, research suggests, is similar to people who have experienced chronic stress. Other research has found effects on the immune system, learning and memory.

We know a lot about the small but serious health risks associated with the pill – things like stroke and blood clots. Why have we been kept in the dark about the effects on the brain?

Until very recently, there has been little research. And the research that is out there doctors often aren’t aware of because it isn’t being published in the medical journals they look at, but rather in psychology and neuroscience journals. Then society has taboos about talking about it. The best defence against the sexist notion that women’s hormones make them less rational than men seemed to be to deny hormonal involvement in the brain. And the pill is so useful, no one is motivated to examine it too critically. But our hormones, especially our sex hormones, are a key part of what creates the experience of feeling like ourselves. And talking critically about the pill doesn’t mean that the benefits aren’t going to outweigh the cost. It’s not antithetical to women’s rights to talk about this stuff.

Some women are going to experience big changes while others are going to have none Sarah E Hill

How does the pill potentially affect women’s choice of mate?

Oestrogen is known to nudge women’s preferences when it comes to their romantic partners to favour qualities that are associated with masculinity and higher testosterone: square-cut jaws, broad shoulders and brow ridges, for example. But research suggests pill-taking women – in their state of artificial progesterone dominance and lacking a cyclical oestrogen urge – seem to prefer the faces of men who are less masculine. The implication is that if a woman chooses her partner when she’s on the pill and then goes off it – it might lead to relationship dissatisfaction because she no longer finds herself as attracted to the person. It is a possibility that women should at least keep in mind.

What about the mood changes women report on the pill?

Being on the pill is associated with an increased risk of being diagnosed with anxiety and depression, though there are some women who report the pill stabilises their moods. The association was particulary high in young women aged 15-19 and their suicide risk is also more than twice that of non-pill takers. A dysregulated stress response is likely part of the picture, but another driver seems to be low levels of the neurosteroid allopregnanolone – a breakdown product of progesterone that acts as a natural sedative. In pill-taking women, the research suggests, levels are significantly lower than in naturally cycling women. Women starting the pill should keep a mood journal or tell a friend to keep an eye on them, especially if they have a personal or family history of mood-related symptoms. For young women, the pill may not be worth the trade-off if it is being used for reasons other than to prevent pregnancy – for example to clear acne or regulate periods.

How big are the effects you are talking about?

They are relatively small. But at an individual level, some women are going to experience big changes while others are going to have absolutely none. Each woman should know about the research that’s out there so they can monitor themselves and troubleshoot.

There are over 100 different formulations of pill along with multiple other hormonal birth control options. Should we treat them the same?

The signalling is going to be different. Progestins divide into four different generations each with a somewhat different molecular structure. Each pill can also have a different ratio of progestin to artificial oestrogen. Right now, the research that exists rarely divides women based on the type of progestin they are on, let alone the ratios. We need to push science to start looking at this.

Should women seek alternatives to the pill?

I am not making recommendations to any individual woman. Each knows what her own life goals are and how her body feels. But I do advise women to be strategic in their use of the pill. Understand the psychological trade-offs and then decide whether or not you want to use it. For a lot of women, the answer is still going to be yes, but now they choose knowing that information. With all the different formulations out there, I am confident women who want to use the pill are going to be able to find one that works for them. For women who are looking for something which doesn’t have hormones in it, the copper IUD is one of the most failsafe.

Are you concerned that raising these issues might turn women to less effective methods of birth control that could leave them with unwanted pregnancies?

It is the last thing I want to have happen. I’m hoping we can use this moment to educate ourselves, but still recognise how important it is to protect ourselves from unwanted pregnancy. Not being pregnant when you don’t want to be pregnant – for me the benefits of that far outweigh almost any brain changes the pill can cause.

You say the issue of birth control is not yet solved. So what would solve it? Better pills, a male pill, or something new entirely?

Ideally we want contraception that’s really safe and effective that doesn’t monkey with anyone’s brain. Maybe that means more investment in the copper IUD. Or maybe it is something yet to be invented. But we need the imagination of the best researchers on this issue. The pill works so well and is so easy, we’ve become complacent. And in the meantime, we need more and better research to understand what we’ve got.

• How the Pill Changes Everything: Your Brain on Birth Control is published by Orion (£19.99). To order a copy go to guardianbookshop.com or call 020-3176 3837. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99