Tens of thousands of Britons taking a common pill to treat restless legs could be at risk of suffering aggressive outbursts, experts have warned.

Behaviour changes on the drug, called ropinirole, have previously been documented - including high-risk tendencies like gambling and hypersexuality.

But now drug chiefs have added aggression to the medication's patient safety information leaflet that warns of side effects, the Daily Mail has learned.

'On July 8th, following a routine European-wide safety review, aggression was added to the warning...alongside other recognised effects such as pathological gambling, hypersexuality and compulsive spending,' said Robert Bradshaw, superintendent pharmacist at Oxford Online Pharmacy.

The drug, sold under brand names Adartrel and Requip, is a dopamine agonist which works by mimicking the behaviours of the hormone dopamine, which helps regulate movement and mood.

It is prescribed to alleviate symptoms of restless leg syndrome, a condition that causes an irresistible urge to move the limbs and affects around one in 10 adults.

The problem is thought to be linked to a disruption in the use of dopamine within the brain.

Ropinirole is also commonly given to Parkinson's patients, who suffer tremors as a result of diminishing dopamine.

Graphic courtesy of the Center for Vascular Medicine. It shows the various sensations that a person with RLS feels as they try to fall asleep

The curious behavioural side effects of the medication are thought to occur as a result of dopamine's effect on mood.

Dopamine agonist drugs can over-stimulate feelings of pleasure triggered by the hormone, giving rise to impulsive behaviour. 

Drug experts have welcomed the addition of aggression to the official list of side effects.

Dr Leyla Hannbeck, Chair of the Independent Pharmacies Association, said it's long been known among pharmacists that aggression is a 'side effect to watch out for', albeit rare.

The latest development comes after 20 female patients accused doctors of failing to warn them about serious side effects that led them to seek out risky or sexual behaviours, according to a 2025 BBC investigation.

While some women reported an uncontrollable urge to spend or gamble, racking up debts of more than £150,000, others said they sought out risky sexual behaviours.

One woman, Claire, was prescribed the drug after developing restless legs during pregnancy.

At first, the drug significantly improved her symptoms, but after a year the new mother began feeling unprecedented sexual urges.

Sally Gardner, a best-selling children's author, spiraled into reckless spending after being prescribed dopamine agonist drugs for restless leg syndrome which saw her blow more than £500,000

Following a European-wide safety review, aggression has now been explicitly added to the warning on impulse control disorders associated with Adartrel

She claimed she was not warned by doctors of any side effects, despite the UK medicines regulator stating that the body had warned the pills can increase libido and harmful behaviours.

'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,' she said.

It took years of putting herself in increasingly dangerous situations – where she would expose her breasts in public – for her to connect the urges with the medication.

Her sexual deviance disappeared almost immediately when she stopped taking ropinirole.

Impulsive behaviours, including addictive gambling, excessive eating or spending and an 'abnormally high sex drive or an increase in sexual thoughts or feelings or aggressive behaviours' are other listed side effects.

The current patient information leaflet for ropinirole also makes specific reference to changes in sexual interest on five occasions.

Restless leg syndrome is thought to affect around one in 20 adults – and women are about twice as likely to suffer as men.

Symptoms include a strong need to move your legs, uncomfortable tingling, throbbing or itching and pain and are usually worse at night.

Treatment depends on what's causing the condition, which has been linked to pregnancy, iron deficiency, kidney disease and some medications.

Dopamine agonists are usually only prescribed when other medicines such as painkillers or anti-anxiety medications have not worked.

Bradshaw added: 'Ropinirole has been used in the UK for over 20 years and has been an important and well-established treatment option for both Parkinson's disease and restless legs syndrome.

'Over that time it has helped many thousands of patients manage difficult and disruptive symptoms, and it remains a valued part of standard care today.'

Patients taking the drug who notice any change in their mood, behaviour or impulses are advised not to stop taking the medication without advice from a GP.

GSK has been approached for comment.