Canadian soldiers who took a military-issued anti-malarial drug held a town hall in Ottawa, Ont., on Sunday to discuss both their concerns that the drug may have caused incapacitating mental health side-effects and a class-action lawsuit they are set to launch.

Nearly 1,000 Canadian veterans have joined the lawsuit, which will sue the federal government, claiming that the anti-malarial medication mefloquine has left them suffering from depression, night terrors, aggressive behaviour and suicide.

Philip Brooks, a retired air force captain who joined to lawsuit, was on the drug for two weeks before his mission to the Congo was cancelled.

He said he received no warning about any potential side effects of the drug, but immediately after taking it, began having vivid nightmares and experiencing a sense of doom.

“I went down and couldn’t recover,” said Brooks, who had no previous history of depression or anxiety.

Mefloquine is highly effective against malaria and was first given to the Canadian troops heading to Somalia in the early 1990s, even before it was approved by Health Canada.

But some experts believe it may be linked to long-term effects that often mimic the symptoms of post-traumatic stress disorder.

“In some people, the drug either accumulates or acts in some way in the brain tissue to cause damage,” said Dr. Remington Nevin, the executive director of The Quinism Foundation, a non-profit dedicated to supporting research and education of the effects of mefloquine.

She added that related drugs “have actually been known to cause permanent injury and destruction in diverse areas in the brain stem or limbic system.”

A Canadian Armed Forces study in 2017 found that there is “limited evidence” that the drug causes “long-lasting and permanent neurological and psychiatric adverse events.”

In a statement, the Canadian Armed Forces said that it takes the health and well-being of its members seriously. It said that it stopped using mefloquine in 2017 as its preferred anti-malarial drug as a precaution.

Mefloquine has been the subject of lawsuits before. In the United States, veterans who claimed to have been harmed by the drug are receiving disability compensation. Earlier this year, lawyers for a former American army sergeant argued in court that the drug caused psychosis that was linked to his massacre of 16 Afghan villagers in 2012.

The sergeant, Robert Bales, was sentenced in 2013 to life in prison without parole.

With files from CTV Medical Correspondent Avis Favaro and senior producer Elizabeth St. Philip