When a 15-year-old boy was shocked -- to death -- by a Taser stun gun in Michigan in 2009, Amnesty International called for rigorous testing of the weapon, which delivers an initial 50,000-volt shock.

Forensic Science International

"There have to be ways of restraining an unarmed teenager other than using electro-shock weapons," the organization said in a statement. "Taser guns are not the safe weapons they are portrayed to be. A full investigation into their safety needs to be carried out before more people suffer the consequences of their misuse."

Now, two research papers are weighing in on the issue, and the consensus is pretty clear: Tasers alone can (and do) go beyond subduing and incapacitating. (Amnesty reported in February that, since 2001, 500 people have now died from Tasers either during arrest or in jail.)

In one study, published in the journal Circulation, a cardiologist at the Indiana University School of Medicine has found that out of eight cases where U.S. police tased men (aged 16 to 48) to the point of loss of consciousness, seven suffered cardiac arrest and died.

Prof. Douglas Zipes says his study is the first peer-reviewed scientific research to show that being shocked with a Taser can cause cardiac arrest and death. And while some had high alcohol levels or heart disease, he tells the Vancouver Sun, "Whatever it is caused the cardiac arrest exactly at the same time the Taser shock was delivered, and that coincidence starts to stretch the imagination."

Because Zipes had been called as an expert on the effect of Tasers in all eight cases he studied, and was paid handsomely at $1,200 an hour to testify, Taser International -- the maker of Tasers -- accuses Zipes of publishing a self-serving article from which he stands to profit.

Zipes' reply: "There is no incentive to write a paper -- for which I did not get paid -- that educates people about the use of Taser and its potential for cardiac arrest...When you cannot attack the science, you attack the scientist."

Meanwhile, a case study out of France reports on a 27-year-old man who was tased while drunk and resisting police requests for his ID. The police managed to fire a barbed Taser dart into the side of the man's head, which somehow went unnoticed until the man complained of a persistent headache and was taken to the hospital.

It turns out that, as seen by a CT scan, the dart "penetrated the frontal part of the skull and damaged the underlying frontal lobe," according to the team's report in Forensic Science International.

Fortunately, the team was able to surgically remove the dart from the man's skull and brain without any additional lesions, and he was discharged a week later without any known infections or neurological complications.

The team's conclusion? "This case study underlines the potential risk induced by the use of Taser stun gun. Although generally regarded as a safe alternative, serious injuries have ... been reported and questions regarding the safety of the device remain unresolved."