The contraceptive pill has been sparking controversies since it became available in the UK in 1961.

Despite research linking it to everything from breast cancer and blood clots to low libido and weight gain, it remains the most popular form of contraception for women today and is taken by more than 100 million women worldwide.

But countless studies have identified strong links between taking the pill and poor mental health, and a new BBC Two documentary sheds light on the severity of the problem, revealing how it’s left some women suffering from depression and experiencing suicidal thoughts.

The hormone in the pill that has been linked to prompting psychiatric complications is called progesterone, which is found in both the combined pill and the mini pill.

Studies have linked the hormone to depression, anxiety and low mood, but researchers have yet to find an ethical way to prove cause and effect because this would involve distributing placebo pills to study subjects, which could lead to unwanted pregnancies, the moral complexities of which are obvious.

A survey conducted by the team behind the documentary, titled The Contraceptive Pill: How Safe Is It?, found that one in four women taking the pill said it had negatively affected their mental health.

Danielle, 31, is one of these women. Speaking in the Horizon episode, she recalls how her side effects were “completely debilitating and terrifying”, adding that she’d never experienced poor mental health prior to taking the pill.

“I went from being fine to having suicidal thoughts within six months,” she adds.

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Grazia’s editor-in-large Vicky Spratt had a similar experience, recalling how her mental health deteriorated soon after she began taking the pill aged 14, leaving her suffering from depression and regular panic attacks.

“I remember thinking ‘if this is what the rest of my life is going to be like, I don’t want to live it',” Spratt, 30, says in the documentary.

At the time, Spratt was prescribed antidepressants and cognitive behavioural therapy, but she claims that not a single doctor thought to make the connection between her deteriorating mental health and her contraceptive pill.

After some extensive Googling, she came across research online that linked the pill to depression and decided to stop taking it to see if it would make a difference; she felt better “within weeks”.

In the documentary, we see Spratt interview the Copenhagen-based professor Øjvind Lidegaard, who has access to a unique database of medical records because in Denmark, every single person’s data is logged in a central system.

This enabled him to look at the records of more than one million Danish women aged between 15 and 34 over a sixteen-year period, producing two studies on the topic of mental health and hormonal contraception.

One found that women taking the pill - either the combined pill or the progestogen-only pill - were more likely to be prescribed an antidepressant than those not on hormonal contraception. The difference was particularly noticeable for young women aged between 15 and 19 on the combined pill. Another found a link between hormonal contraception, suicide attempts and suicide.

Lidegaard's findings were confirmed by a separate Swedish study of more than 800,000 women that had been published in March.

Despite the number of women experiencing poor mental health as a result of the pill, Spratt says little is being done to combat this in the UK due to a lack of data and an apparent reluctance from the NHS to take measures to rectify this.

She has been investigating the links between the contraceptive pill and mental illness for nearly two years and, after conducting a series of freedom of information requests, found that this was not something the NHS had any data on.

This means that the NHS is not monitoring women who are currently taking hormonal contraception and being treated for mental health issues, at least not in the same way that Danish medical officers are.

It also means that women may not be fully informed about these possible side effects prior to taking the pill, with no mention of depression or anxiety on leaflets distributed to women in sexual health clinics. Instead, they list “mood changes” as a possible side effect.

“What does that even mean? It could refer to anything,” Spratt tells The Independent.

“I think the NHS is sticking with ‘mood changes’ because they’re nervous about women being deterred from taking the pill and unwanted pregnancies rising as a result.

“It surprises me that despite known links between progesterone and depression, the NHS aren’t looking into it and, as a result, we don’t know how many women are affected by this in the UK.”

Spratt describes the seeming reluctance as “odd”, adding that this is why she decided to take part in the BBC documentary.

“We just need to do the research and collect the data so doctors are able to properly inform women about the potential risks of taking the contraceptive pill.”

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Dr Zoe Williams, who presents the documentary, explains that all medicines have potential side effects, but the pill is different to most because it is taken by women who are well, rather than to prevent or treat illness or disease.

“Therefore side effects that significantly impact on quality of life, in a negative way, are not acceptable,” she tells The Independent.

“Especially when there are so many types of pill and alternatives methods for contraception available.”