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 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.
Source: BBC
