From Korean Air flight crews to U.S. police and corrections officers to beat cops in Britain, the orders are pouring in for stun guns (search) made by Taser International Inc (search).

The Scottsdale company even recently launched a metro Phoenix ad campaign urging private citizens to arm themselves with the weapons, which temporarily paralyze people with a 50,000-volt jolt.

Yet while Taser's stock has soared with the booming business, concerns are growing about whether the shock-inducing guns are truly as non-lethal as advertised.

In a report being released Tuesday, Amnesty International (search) says stun guns are being abused by police and wants more scientific study done to determine whether the devices are safe.

Amnesty says at least 74 people have died in the United States and Canada in the past four years after being shocked with Tasers.

The group also says officers have turned stun guns on the mentally disturbed, children and the elderly.

"Not only do we not know the impact of these weapons on human beings under various conditions, we are also concerned about the gratuitous use of these weapons," said Gerald Le Melle, deputy executive director of Amnesty International USA.

A Defense Department-sponsored report also calls for more testing, and some health professionals are expressing concern that the potential for cardiac arrest could be too high.

Amnesty worried that "the deployment of Tasers, rather than minimizing the use of force, may dangerously extend the boundaries of what are considered 'acceptable' levels of force."

Similar to a device first developed the 1970s, the Taser became available to consumers in the early 1990s and is now used by more than 6,000 law enforcement agencies worldwide as well as the U.S. military, which has used them in Iraq and Afghanistan and ordered nearly $2 million worth of stun guns and accessories this summer.

Taser officials bill the guns, which shoot two barbed darts whose current can penetrate up to two inches of clothing, as among the safest ways of subduing violent people in high-risk situations. Tasers have a range of up to 21 feet and can also shock on contact, like a cattle prod.

"We get e-mail from police every week ... thanking us for developing a weapon so they didn't have to shoot somebody," said company chairman Phil Smith. "We're saving lives every day and cops love them."

Phoenix police officers credit Tasers with helping police shootings drop by more than half and fatal shootings by 31 percent last year.

"We've seen them reduce injury to suspects ... who in the past we would have had to strike multiple times with fists or batons," said Sgt. Randy Force, a department spokesman.

While not opposed to stun guns in principle, Amnesty International wants law enforcement to stop using Tasers until scientific evidence can show they don't kill.

In a majority of Taser-related fatalities, coroners have attributed the cause of death to heart problems, drug overdose or asphyxiation. But some medical experts believe Taser shocks may exacerbate a risk of heart failure in cases where people are agitated, under the influence of drugs or have underlying health problems.

"If I hit the heart or create electricity in the wrong time of the (beat) cycle, it could send the whole heart into an electrical tailspin," said Dr. Kathy Glatter, an electrophysiologist and assistant professor of medicine at the University of California-Davis.

Amnesty International claims stun guns are being fired too often where the use of force is unacceptable. In many of the deaths it cites, the person was shocked multiple times or subjected to other forms of force, like pepper spray, batons or hogtying.

In one such case, 47-year-old James Borden died in an Indiana jail after being stunned at least half a dozen times with a Taser. An autopsy report listed his cause of death as consistent with cardiac dysrhythmia, secondary to abnormal thickening of the heart, drug intoxication and electrical shock.

Most of the deaths documented by Amnesty occurred this year, just as sales picked up amid positive reviews.

A Department of Defense-ordered study cited by Taser company officials in defending their technology does, however, also recommend more research on how Tasers affect sensitive or intoxicated people.

The study, done by an Air Force laboratory for the Joint Non-Lethal Weapons Directorate, found that when used as intended, Tasers are safe. But it also said that, although uncommon, some severe unintended effects might occur after a shock. The military has not released the entire study, only an abstract.

A military spokesman said the study's author, James Jauchem, couldn't be reached for comment. But the center that employs Jauchem did say that it does not endorse or approve use of the systems it studies.

Taser International, meanwhile, says it has documented 600 cases of lives saved with Taser devices and that it has not settled or lost any lawsuits involving deaths or injuries related to the guns.

"Fact is ... this thing has been around for 30 years, never ever has a Taser been listed as the primary cause of death," said Smith, the company chairman.