Ketamine provided almost instantaneous relief to bipolar patients suffering severe depression and suicidal thoughts, according to researchers at the National Institute of Mental Health.

Depressive symptoms have significantly improved 40 minutes after researchers injected a single dose of ketamine, sometimes sold illegally under the name "Special K" as a popular recreational club drug, in bipolar patients.

Furthermore, researchers found that patients who were given the drug, which is also used as a horse tranquilizer, had improved symptoms that lasted over 3 days.

Bipolar disorder is a condition in which individuals experience severe "mood swings" between mania and depression. The episodes of severe depression or elevated mood can last for days or months, and the risk of suicide is high.

While antidepressants are commonly prescribed to treat or prevent depressive episodes, they are not effective in treating everyone and many people with the disorder still continue to experience periods of depression even while being treated, according to researchers.

Many patients need to try several types of antidepressants before finding one that works for them, and existing treatments can take several weeks before patients begin to feel relief.

The latest findings, published in the journal Biological Psychiatry, confirm previous findings that a single dose of ketamine produced rapid antidepressant effects in depressed patients with bipolar disorder.

Researchers injected a single dose of ketamine and a single dose of placebo to a group of patients on two different days, two weeks apart. Investigators then carefully monitored and repeatedly scored patients' depressive symptoms and suicidal thoughts.

They found that depression symptoms in patients had dramatically improved within 40 minutes of administering the drug, and remained improved over 3 days. While 79 percent of the patients improved with ketamine, none of the patients in the placebo group reported improvement.

More importantly, patients receiving the ketamine injection had significantly reduced suicidal thoughts after an hour of administering the drug.

Researchers noted because bipolar disorder is one of the most lethal of all psychiatric disorders, the latest findings could have a major impact on public health.

"Our finding that a single infusion of ketamine produces rapid antidepressant and antisuicidal effects within one hour and that is fairly sustained is truly exciting," Dr. Carlos Zarate said in a statement.

"We think that these findings are of true importance given that we only have a few treatments approved for acute bipolar depression, and none of them have this rapid onset of action; they usually take weeks or longer to have comparable antidepressant effects as ketamine does," he added.

Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist and works by blocking the actions of NMDA.

"Importantly, confirmation that blocking the NMDA receptor complex is involved in generating rapid antidepressant and antisuicidal effects offers an avenue for developing the next generation of treatments for depression that are radically different than existing ones," Zarate added.