By Matthew Davis
BBC News, Washington
With electronic stun guns now being used by a growing number of UK police forces, BBC News examines the controversy in North America, where a series of deaths have put Tasers under fresh scrutiny.
Robert Bagnell died after police shocked him with 50,000 volts
When Robert Bagnell died in Canada last June, his family were told he had suffered a probable cocaine overdose.
The truth emerged in fits and starts, however. They eventually learned that police had jolted him with 50,000 volts of electricity from a Taser gun.
Officers said they had to subdue Mr Bagnell to save him from a fire.
But a year on, questions over his death are still unanswered, while concerns over the safety of stun guns refuse to go away.
The "less-than-lethal" weapons have been involved in 74 deaths in the United States and Canada, according to Amnesty International.
These reports clearly indicate that the Taser technology, while not risk-free, is among the safest use-of-force options our law enforcement officers have
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It also claims Tasers have been used gratuitously - against children or the elderly or on people posing no real threat.
In a sign of mounting concern over the effects of the weapons, Chicago police halted deployment of Tasers in February after a teenager suffered cardiac arrest and a man died after being stunned.
For Patti Gillman - Robert Bagnell's sister - too many mistakes are being made.
"It is far more than a coincidence that my brother was shot by a Taser, then died of a heart failure.
"We believe he was having a seizure at the time police stunned him. But he was unarmed, not posing a threat.
"The first response should have been medical attention, not assaulting him with a weapon."
Vancouver police say they are still awaiting a final report into the death, but believe stun guns are safe to use.
Tasers 'relatively safe'
The widely used M26 Taser gun delivers a 50,000-volt shock through two barbed darts, which incapacitate people for five seconds.
The effects of the Taser gun are felt by officers training with it
Rick Smith, CEO of Taser International, says that while it is "not risk-free" it is "among the safest use-of-force options our law enforcement officers have".
He blames Amnesty's report for a downturn in business which has seen Taser's stock lose more than 60% of its value in the first quarter of this year, wiping $1.4bn off its market value.
In a statement he said: "Anyone living in the real world in which law enforcement officers worldwide have to make split-second life or death decisions knows that Amnesty International's report and position is out of step with the needs of law enforcement concerning our proven life-saving technology."
A report in March by the US Potomac Institute for Policy Studies said current research suggested stun guns were "relatively safe" when used appropriately.
It noted that in all the cases in the Amnesty report, other factors like drug use and pre-existing heart conditions could also have led to the deaths.
But the institute called for more research, particularly in understanding exactly how the weapons work on the body - and what the long-term side effects could be.
'Learn from mistakes'
Meanwhile, the US National Institute of Justice (NIJ)is funding several studies in to what is describes as "less lethal weapons issues".
Last September it gave a $490,000 grant to the University of Wisconsin to study the effects of electro-muscular devices and how electrical currents move through the body.
It is also funding a International Association of Chiefs of Police study on best practice in use of the weapons.
The moves come as a number of UK police forces - where officers are not routinely armed - are embracing the weapons.
In May, a Police Review survey of 100 officers found 80% thought that all officers on frontline duties should be issued with stun guns.
More than half the public also want to see Tasers being deployed, according to the magazine, which polled 1,000 citizens.
The UK Home Office believes Tasers are safe to use, but says their use should be restricted to firearms-trained officers.
Patti Gillman, still searching for answers on her brother's death, sounded a cautionary note.
"The Taser issue is controversial in Canada only in certain circles, but is mostly ignored by the mainstream public," she said.
"But I suppose it's only background noise to most, until it enters your life as it did ours.
"I hope the British police, having given Canada and the US a bit of a head start, will tread carefully and learn from our mistakes."