Depression is not a laughing matter, but that may just be the trick to treat the condition. 

Researchers at Washington University in St. Louis say they've discovered that laughing gas, or nitrous oxide, could be an effective treatment for those suffering from severe depression.

This hypothesis turned true when the researchers tested it in a recent study. The researchers gave a group of 20 patients two rounds of treatments - some got the laughing gas while others were given a placebo, reported CBS News.

About two-thirds of the patients who received the laughing gas reported improvements in their symptoms of depression. Only one-third of the patients in the placebo group felt their symptoms were improving. 

"The nitrous oxide treatment improved it above and beyond the placebo," Dr. Peter Nagele, assistant professor of anesthesiology at Washington University's School of Medicine, said in a podcast produced by the school. "This was fairly rapid, so at two hours. But our primary endpoint when we measured everybody - we asked the patients to come back the next day - was sustained to a day. " 

The researchers also found that laughing gas has less side effects than similar drugs such as katamine, reported CBS News. Both are used as a sedative by dentists, but katamine is much stronger.

The findings were published Tuesday in the journal Biological Psychiatry.