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The strongest link was for depressed patients prescribed Effexor (venlafaxine) or antidepressants called serotonin reuptake inhibitors (SSRIs), the British study found. SSRIs include citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac), paroxetine (Paxil) and sertraline (Zoloft).
Also, the study was observational, and "we did not demonstrate a causal association between antidepressants and mania and bipolar disorder," said lead researcher Dr. Rashmi Patel, of the department of psychosis studies at King's College London's Institute of Psychiatry, Psychology and Neuroscience.
"If you are taking antidepressants and are concerned that you might be experiencing adverse effects, it is important to seek medical advice to review your medication and to not stop your treatment suddenly, as this may result in withdrawal symptoms," Patel said.
Major depression is one of the most common mental disorders in the United States, According to the U.S. Centers for Disease Control and Prevention, about one in 10 Americans aged 12 years and over takes antidepressant medication.
For the study, Patel and colleagues studied the medical records of more than 21,000 adults treated for major depression in London between 2006 and 2013.
SSRIs were the most commonly prescribed antidepressants (35.5 percent), the researchers said.
Effexor, a dual-acting drug used to treat both depression and anxiety, was taken by less than 6 percent of patients. Fewer than 10 percent took mirtazapine (Remeron) and fewer than 5 percent used tricyclics (Elavil).
Nearly 1,000 patients were diagnosed with bipolar disorder or mania during the follow-up period of roughly four years.
"We found that antidepressants were widely prescribed and associated with a small increased risk in developing mania and bipolar disorder," said Patel.
This association was particularly strong for SSRIs and Effexor. These drugs seemed to increase the risk 34 percent to 35 percent, the researchers said.
According to the U.S. National Institute of Mental Health, bipolar disorder, also known as manic-depressive illness, causes unusual shifts in mood, energy, activity levels and the ability to carry out everyday tasks.
The study authors noted that people with undiagnosed bipolar disorder may be more likely to seek treatment when in the depressive stage of the illness, which could help explain the link between antidepressants and later bipolar behavior.
The report was published Dec. 15 in the online journal BMJ Open.
Dr. Ami Baxi, interim director of inpatient and emergency psychiatry at Lenox Hill Hospital in New York City, said, "As the prevalence of depression increases, more and more antidepressants are being prescribed and patients often ask about the risks associated with these drugs."
In this case, however, it is difficult to say that these medications cause bipolar disorder, since several risk factors related to underlying bipolar disorder were not assessed in this study, said Baxi, who was not involved with the study.
This research indicates a correlation of antidepressant treatment and manic episodes without reviewing preexisting risk factors of developing bipolar disorder, she explained.
"For patients who are concerned about this risk of conversion to bipolar disorder, the results of this study should encourage a discussion with your doctor regarding the benefits of the antidepressant and your risk factors for developing bipolar disorder before making any changes in medications," she said.
Patel agreed and said better ways of identifying depression patients who may be at risk of developing bipolar disorder need to be developed.
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