Bipolar disorder patients may be more likely to get Parkinson's, a major review of evidence has suggested.
Scientists uncovered the link after they analysed the results of 160 trials involving 4.3million participants.
Patients with the mood disorder were more than three times more likely to end up with Parkinson's, according to the findings.
Scientists at the Faculdade de Medicina da Universidade de Lisboa in Portugal did not prove bipolar disorder causes the crippling condition.
However, they believe drugs bipolar patients have to use for years to control their disorder could be to blame.
Another suggestion is that bipolar disorder may lower levels of dopamine, and a lack of dopamine is the main cause of Parkinson's.
People with bipolar disorder are up to three times more likely to get Parkinson's
Charities today welcomed the evidence published in JAMA Neurology - but warned the risk of a bipolar patient developing Parkinson's is still slim.
Dr Patrícia Faustino and colleagues combined the results of seven existing studies that investigated the prevalence of Parkinson's in bipolar patients.
One was conducted in the UK, involving more than 3,000 people, and another took place in the US and had more than 3.4million people.
The findings suggest that a previous diagnosis of bipolar disorder was linked with a 3.35 increased likelihood of Parkinson's.
A second analysis suggested bipolar patients face a 3.21-fold higher risk of getting the condition.
Bipolar disorder is a brain disorder which causes unusual and often sudden changes in mood and energy levels.
WHAT ARE THE SYMPTOMS?
Moods of those with bipolar disorder range from periods of extreme elation and energy (known as a manic episode) to periods of extreme somberness and lack of energy (known as a depressive episode).
HOW IS IT DIAGNOSED?
According to the International Bipolar Foundation, sufferers are diagnosed with rapid cycling if they have four or more manic, hypomanic, or depressive episodes in any 12-month period.
This severe form of the condition occurs in around 10 to 20 percent of all people with bipolar disorder.
WHAT CAUSES IT?
Currently it is unknown what is the cause of bipolar disorder, which affects around 5.7 million US adults aged 18 or older.
Scientists say genetics could play a role or that those with a a family history of bipolar disorder are more likely to have it.
The researchers said the link may be explained by the drugs bipolar patients have to take.
Bipolar causes extreme highs and lows, called manic and depressive episodes, which can interfere with daily life.
It's usually diagnosed in teenagers, meaning patients are forced to take medication every day for the rest of their life to try and prevent a switch in mood.
The most common mood stabilisers prescribed in the UK contain lithium, taken for at least six months.
It is lithium, which, over long periods of time, may induce Parkinson's, the researchers said.
About seven per cent of people with parkinsonism - a term that covers several conditions including Parkinson’s - have developed their symptoms following treatment with particular medications, according to Parkinson's UK.
Any drug that blocks the action of dopamine, referred to as a dopamine antagonist, is likely to