Patients prescribed drugs for movement disorders – including restless leg syndrome (RLS) – say doctors did not warn them about serious side effects that led them to seek out risky sexual behaviour.

Twenty women have told the BBC that the drugs – given to them for RLS, which causes an irresistible urge to move – ruined their lives.

A report by drugs firm GSK – seen by the BBC – shows it learned in 2003 of a link between the medicines, known as dopamine agonist drugs, and what it described as “deviant” sexual behaviour. It cited a case of a man who had sexually assaulted a child while taking the drug for Parkinson’s.

While there is no explicit reference to this side effect in patient leaflets, the UK medicines regulator told us there was a general warning about increased libido and harmful behaviour. GSK says a risk of “altered” sexual interest is also referred to in the leaflets.

Some of the women who described being drawn to risky sexual behaviour told us they had no idea of what was causing it. Others said they felt compelled to gamble or shop with no history of such activities. One accumulated debts of more than £150,000.

Like many women, Claire first developed RLS during her pregnancies. The relentless need to move was often accompanied by sleeplessness and a crawling sensation under her skin.

The condition persisted after giving birth and she was prescribed the dopamine agonist drug Ropinirole. She says she was not warned by doctors of any side effects. It initially worked wonders for her RLS, she says, but after a year or so she began feeling unprecedented sexual urges.

“The only way I could describe it is it was just deviant,” she tells us – using that word without any knowledge of the GSK research which had established a link with such behaviour.

Claire says she began leaving her house in the early hours of the morning to cruise for sex. Wearing a see-through top and jacket, she would flash her chest at any man she could find. She did this regularly, she says, and in increasingly dangerous locations, despite having a partner.

“There remains an element in your head that knows what you’re doing is wrong, but it affects you to the point that you don’t know you’re doing it.”

Claire says it took years to connect these urges with her medication – and they disappeared almost immediately when she stopped taking it. She feels complete “shame” and is “mortified” at the danger she placed herself in.

Impulsive behaviours, including gambling and increased sex drive, have long been listed as side effects in medicine leaflets for dopamine agonist drugs – and are thought to affect between 6% to 17% of RLS patients taking them, according to health guidance body NICE. A “common” side effect of any medicine is considered to only affect 1% of people who take it, according to the NHS.

The drugs – which are also used to treat schizophrenia – work by mimicking the behaviour of dopamine, a natural chemical in our brains which helps regulate movement. It is known as the “happy hormone” because it is activated when something is pleasurable or we feel rewarded.

But agonist drugs can over-stimulate these feelings and under-stimulate the appreciation of consequences – leading to impulsive behaviour, according to academics.

The cases of what the GSK report from 2003 described as “deviant behaviour” involved two men who were prescribed Ropinirole for Parkinson’s disease. In one, a 63-year-old-man sexually assaulted a seven-year-old girl, leading to a custodial sentence.

The documents said the perpetrator’s libido had increased significantly from the start of his treatment with Ropinirole and his “libido problem subsequently resolved” after his dose was reduced.

In the second case, a 45-year-old man carried out “uncontrolled acts of exhibitionism and indecent behaviour”. His sex drive was reported to have increased prior to being prescribed Ropinirole but his urges “intensified” after the treatment.

Prevalence rates of what GSK calls “deviant” sexual behaviours caused by the drugs are unknown and tend to be under-reported by those who experience them, according to Valerie Voon, a professor of neuropsychiatry at the University of Cambridge.

“There’s a lot of stigma and shame attached to it, and people don’t realise that it’s associated with a medication,” she says.

Prof Voon believes risky sexual behaviours – beyond a purely increased libido – should be specifically warned about and screened by the NHS, because their impact can be “devastating”.

RLS is believed to affect about one in 20 adults – and women are about twice as likely to suffer as men.

The 20 sufferers we spoke to say not only had doctors failed to tell them of the potentially serious side effects of the drugs, but also failed to review the impact of the medication on their bodies subsequently.

Sarah was in her 50s when she was prescribed another dopamine agonist drug made by a different manufacturer.

“Previously I’d have had no interest if Brad Pitt walked in the room naked,” she says. “But it turned me into this raging woman who kept taking sexual addiction further.”

Sarah began selling used underwear and videos of sex acts online – and organising telephone sex with strangers. She also began shopping compulsively – ending up with £30,000 of debt.

To combat the effects of the dopamine agonist, she began self-medicating by taking pain-relieving opioids and sleeping pills. She ended up being admitted to rehab – but that meant her driving licence was taken away and she lost her job.

“I turned to things that weren’t healthy – I knew that the behaviour wasn’t me, but I couldn’t control it,” she tells the BBC.

• If you have more information about this story, you can reach Noel directly and securely through encrypted messaging app Signal on: +44 7809 334720, by email at noel.titheradge@bbc.co.uk, external or on SecureDrop

A third woman, Sue, says she was prescribed two different dopamine agonist drugs without being warned of compulsive behaviour side effects on either occasion. She even mentioned recent gambling behaviour when the second drug was prescribed, she says. She went on to rack up debts of £80,000.

“The effect on my family was horrific – it was life-changing money to lose,” she says. “But at the time I didn’t know it was no fault of my own.”

A class action was brought against GSK in 2011 by four sufferers of Parkinson’s disease – the BBC has learned. They said Ropinirole led to gambling debts and broken relationships.

They also complained that despite a link between such behaviours and the drug having been established in medical studies as early as 2000, GSK had failed to include any warnings in its product literature until March 2007. The class action was settled but GSK denied liability.

Cases of serious side effects have also been reported in other countries, particularly in relation to the use of drugs for Parkinson’s disease.

In France, a court awarded damages to a father of two who complained that Ropinirole had given him compulsive homosexual urges, while another man without a criminal record began torturing cats.

In the US, the American Academy of Sleep Medicine recommends the drugs should only be used for short-term treatment, such as end-of-life care.

Many of the women the BBC spoke to also complained that prolonged use of the drugs also worsened their underlying RLS. It meant their dosage had been increased which, in turn, had exacerbated their compulsive behaviour – a process known as augmentation.

Dr Guy Leschziner, a consultant neurologist, says the drugs still play an important role but he believes that drug companies, health authorities and doctors need to better warn patients of these side effects.

“Not everybody knows the kinds of really quite dramatic changes that can occur,” he says.

In a statement, GSK told the BBC Ropinirole had been prescribed for more than 17 million treatments and undergone “extensive clinical trials”. It added the drug had proven to be effective and had a “well-characterised safety profile”.

“As with all medicines, [it] has potential side effects and these are clearly stated in the prescribing information,” it said.

In response to its 2003 research that had found a link with “deviant” sexual behaviour, GSK told us this was shared with health authorities and had informed updates in prescribing information – which now lists “altered or increased sexual interest” and “behaviour of significant concern” as side effects.

The current patient information leaflet for Ropinirole makes specific reference to changes in sexual interest on five occasions – almost exclusively warning about the frequency or strength of such feelings as potentially “abnormally high”, “excessive” or “increase[d]”.

The UK’s Medicines and Healthcare products Regulatory Agency (MHRA), said that while a specific reference to “deviant” sexual behaviour is not included in warnings, such impulses vary and a general warning about activities which may be harmful is included.

It also said that it is important for healthcare professionals to explain the possible risk to patients and not all experience these types of side effects.

The Department of Health and Social Care declined to comment.

Some names have been changed in this article to protect people’s identities.