Pharmaceutical companies are focusing fewer resources on developing drugs to combat mental illness, despite the fact it is a leading health issue and represents a potential massive cash cow, a pair of new studies has found.

Two recent reports published in the journal Science Translational Medicine say drug development for major mental illnesses or psychiatric disorders -- such as schizophrenia, depression or bipolar disorder -- is virtually at a standstill.

"Despite high prevalence and unmet medical need, major pharmaceutical companies are de-emphasising or exiting psychiatry, thus removing significant capacity from efforts to discover new medicines,” states one report, written by Dr. Thomas Insel, director of the U.S. National Institute of Mental Health, and Steven Hyman, of Harvard University.

That's largely due to stringent regulations and approval processes for mental health drugs, the authors note, as well as difficulty with developing medications that can be proven to actually combat mental illness.

"The central problem is clear: Neither vast unmet medical need, nor large and growing markets, nor concerted sales campaigns that attempt to recast 'me-too drugs' as innovative, can illuminate a path across very difficult scientific terrain," wrote Hyman.

Last year, for example, U.K.-based pharmaceutical giants GlaxoSmithKline and AstraZeneca both announced they were cancelling research into new antidepressant drugs.

Dr. Zul Merali, president of the University of Ottawa's Institute of Mental Health Research, said the development shouldn’t come as a surprise.

"The costs have been going up and up and the number of drugs actually being approved for clinical use are not increasing in parallel,” he told CTV’S Canada AM. “So it's a high-risk investment for the pharmaceutical industry."

Merali said it takes an average of 15 years to bring a drug from the initial experimental phase through clinical trials and onto pharmacy shelves, which is a massive commitment of time and finances. And once a product is available to patients, there's no guarantee it will work.

Of the antidepressant medications currently prescribed, one-third of patients respond well, one-third experience mild relief, and the rest feel no relief whatsoever, Merali said.

“We don't really understand how the brain actually works, what goes awry when you have mental illness, and therefore you're not able to target your treatment to that specific circuit in the brain."

In fact, Merali said some of the most exciting developments in mental health drug research have occurred virtually by accident. Ketamine, a drug that was originally developed for use as an anesthetic, was recently found to have a powerful antidepressant effect when used in small doses.

Merali’s team is currently studying that discovery to determine how it can be translated to actual treatment, since future mental-health discoveries may lie more along those lines, he said, as pharmaceutical companies pull back from the massive investment required.