Bipolar disorder used to be more commonly known as manic depression.
Sufferers experience severe mood swings from deep depression to extreme happiness - mania - which can last for weeks or even months.
Campaigners believe a Briton who was executed in China today may have been duped into smuggling heroin while suffering from the disorder.
They had urged the Chinese authorities to assess the mental health of Akmal Shaikh, 53, from Kentish Town, north London.
People who are bipolar may lose interest in life and find it hard to concentrate or to make even simple decisions during periods of depression.
But during manic periods they may feel so full of optimism and energy that it can affect their ability to make good decisions and judgments, according to the Royal College of Psychiatrists.
Sufferers may feel more important than usual, believe strange things about themselves and behave in embarrassing or even harmful ways.
Someone who is bipolar may make plans that are unrealistic or make odd decisions on the spur of the moment.
Psychotic symptoms may develop during a severe episode of either mania or depression.
During a manic episode this can mean the person develops grandiose beliefs about themselves like they are on an important mission or have special powers and abilities.
The information website NHS Choices said a sufferer may feel self-important during a period of mania.
Their thinking may be illogical, they may be delusional and they may do pleasurable things which have disastrous consequences, like spending large sums of money on expensive items.
The human rights group Reprieve said Shaikh had believed he could become a pop star and that his music could bring about world peace.
Robert Westhead, spokesman for the mental health charity MDF The Bipolar Organisation, said: "This type of bizarre behaviour will sound only too painfully familiar to sufferers from this condition and their loved ones.
"Normally at worst it results in people losing their jobs or destroying friendships and deeply embarrassing themselves.
"However, when people are ill like this they are clearly highly vulnerable to being taken advantage of."
Mr Westhead added: "When manic - or high - people often completely lose touch with reality, sometimes experiencing delusions of grandeur and fantastical beliefs about their own abilities.
"This leads them to do extraordinary things they would never contemplate when well.
"For example, someone might quit their job thinking they were about to become a famous novelist or even believe they were the Prime Minister or a top businessman.
"It is easy to see how these sorts of beliefs can result in people getting into terrible trouble, sometimes with the police."
Mr Westhead said bipolar disorder "suspends your normal critical faculties" which may have led to Shaikh making an error of judgment which he would not have made if well.
Marjorie Wallace, chief executive of the mental health charity Sane, said: "People with bipolar illness, which Shaikh was said to suffer from, can have thoughts, beliefs and delusions whereby they are unable to distinguish the outside world from their inner psychotic experience.
"Moreover, the longer the condition is untreated, the less likely a person is to be aware of or admit to having an illness."
She said the Chinese authorities may have overlooked the effect such a condition could have on a person's behaviour.
Around one in every 100 adults will have bipolar disorder at some point in their life, according to The Royal College of Psychiatrists.
It usually starts during or after the teenage years and affects men and women equally.
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