The two U.S. pilots involved in an accidental bombing that killed four Canadian soldiers in Afghanistan last April were on government-issued amphetamines, ABC's 20/20 reports.

The report, which aired Friday night, said the amphetamines, also known as speed or uppers, were standard issue to U.S. Air Force combat pilots to help them stay awake on long combat missions.

Maj. Harry Schmidt, 37, and Maj. William Umbach, 43, say they took the pills an hour before the accidental bombing of Canadian soldiers conducting a live-fire nighttime exercise near Kandahar when they were bombed.

The pilots each face four counts of manslaughter and dereliction of duty for the April 17 bombing, in which four Canadian troops died and eight were wounded.

In an exclusive cockpit tape of the pilots, obtained by ABC News, Schmidt is heard saying he is going to fire in self-defence, despite being told to hold his fire.

"I've got some men on a road and it looks like a piece of artillery firing at us... I am rolling in self-defence."

Next, Schmidt says "Bombs away breaking left. Lasers on."

It was only after he dropped the bomb that Schmidt was told it was not the enemy.

'Go-pills'

The two pilots, who are facing up to 64 years in jail if convicted of all charges, were told by superiors they could be found unfit to fly the mission unless they took the pills, their defence lawyers say.

News of the amphetamines, which the Air Force calls "go pills," comes less than a month before the two pilots are to face an Article 32 hearing that will determine if there is enough evidence to go ahead with a trial.

A joint Canadian-American investigation accused the pilots of not following procedure before dropping the 225-kilogram bomb. The pilots claim they were acting in self defence.

Defence lawyers for the two now say the pills would have been at their "maximum serum level" at the time of the bombing and could have caused them to react too quickly.

"They were not told of the most important warning on every label by every manufacturer: Caution," defence lawyer David Beck said.

The U.S. Air Force has ruled out the "go pills" Schmidt and Umbach took as being responsible for the friendly-fire incident and will not be changing their policy, saying they are essential for combat pilots being sent to Afghanistan and Iraq.

Schmidt's mother, Joan, dismisses the military argument that it was the pilot's choice to take it.

"That is not the truth. I mean if you want to fly, you take the pills. If you don't want to take the pills, you're grounded," she said.

Canada a no-go on the go-pills

ABC said the go-pills were "quietly reintroduced" after being banned in 1992 by then Air Force Chief of Staff Gen. Merrill McPeak.

In Canada, retired Lt. Col. Laurie Hawn, a military analyst, told CTV Newsnet that amphetamines are used only in the U.S. Air Force. The Navy and Marine Corp. do not use the pills.

"And we certainly do not do it and our pilots fly similar lengths of missions in combat," Hawn said. "If you're properly trained and fit, we see no reason to have performance-enhancing drugs."

When asked if he thought the pills could have affected the pilots' judgment, Hawn said, "I'm not doctor and I've never taken any of those kind of drugs, but my gut feel would tell me that anything I'm going to put into my body that's going to alter my thought process or my normal performance is probably not good."

Amphetamines are not approved by the Food and Drug Administration to combat fatigue and are listed by the Drug Enforcement Administration as a Schedule Two narcotic, the same category as cocaine, ABC News reports.

With a report from CTV's Roger Smith