A performance-enhancing drug may improve thinking in patients suffering from depression or bipolar disorder. Erythropoietin (EPO), which is used to enhance performance in sports, gives hope to hundreds of millions of people whose daily lives are affected.

EPO is a hormone, produced mainly by the kidneys, that is vital for the production of red blood cells.

EPO stimulates the production of more red blood cells, which allows the blood to carry more oxygen. This ability explains its appeal as a performance-enhancing drug because it increases the amount of oxygen that is carried to muscles, improving endurance and recovery.

Lance Armstrong admitted he used EPO in a process called "blood doping" as well as other performance-enhancing drugs. Medically, a form of EPO is used for the treatment of anemia.

While most people think of depression and bipolar disorder as conditions which affect mood, they also have a dramatic affect on cognitive function — slowing how quickly and how well the brain functions.

This slow-down in thinking can have serious effects on sufferers, making it more difficult to retain a job, pass an exam, or maintain a relationship.

Danish scientists found that that EPO can help restore cognitive function in patients suffering from these mental disorders.

In two randomized controlled trials, they found that EPO had beneficial effects on patients' completion of a range of cognitive tests, including tests on verbal memory, attention span, and planning ability.

"EPO-treated patients showed a five-times greater cognitive improvement from their individual baseline levels compared with placebo treated patients," said lead researcher Dr. Kamilla Miskowiak.

"This effect of EPO on cognition was maintained six weeks after patients had completed their treatment."

Miskowiak also found that patients who performed poorly in neuropsychological tests showed remarkably greater cognitive benefits when given EPO, which means that doctors may be able to target patients for EPO treatment based on how well they perform on neuropsychological tests.

"We need bigger studies to confirm that the effects we have seen can be replicated, to confirm dosage, frequency of use and so on," she said. "EPO is already used medically, so we know quite a lot about safety.

"Although EPO is generally safe if patients' red blood cell levels are controlled regularly, there are certain groups for whom the risk of blot clots is too high — for example people who smoke or who have previously had blood clots.

"So although these results hold out great promise, EPO treatment is not ready to be rolled out as a treatment just yet and may not be for everyone."

The World Health Organization estimates that around 350 million people suffer from depression, with a further 60 million suffering from bipolar disorder, but the drugs normally used to treat depression and bipolar disorders don't have any major effect on cognition.

Up to 70 percent of patients in remission from bipolar disorder, and up to 40 percent in remission from depression continue to have cognitive problems.

There is currently no effective treatment to tackle cognitive problems in these patients.

About 16 million American are affected by depression in any given year, and is the leading cause of disability in those 15 to 44 years of age. About 11 percent of adolescents have suffered from depression by the age of 18.