It was an ordinary November morning when Howard Swarray went to his local gym in Whalley Range, Manchester. During his regular workout, he started to feel light-headed and suffered an epileptic seizure. Paramedics were called but, as the 41-year-old father of three writhed in agony, so were Greater Manchester police. As a disorientated Swarray struggled with those restraining him, an officer racing to the scene was recorded on police radio saying: "If he's getting aggressive I am sure 50,000 volts will stand him up."

Swarray has no memory of what it felt like to be shot with an electric stun gun but his medical notes recorded that a Taser was used against him five times. Swarray was so heavily sedated with ketamine by an attending doctor in order to transport him to hospital that he spent eight days in a coma. He was subsequently diagnosed with kidney failure.

This was only one of nearly 9,000 incidents in which UK police deployed Tasers in the six years after they were first trialled in 2003. The frequency with which the American-built electronic stun gun is pointed by police – the only people alongside the military who are permitted to use them in this country – is rising steadily. Tasers were last month deployed in the eviction of Travellers from Dale Farm in Essex and against a mentally ill 72-year-old in Cornwall. Last year one was accidentally fired into a 14-year-old girl. They have been fired by police in every corner of the country; and in situations ranging from the Raoul Moat siege to stunning an Alzheimer's sufferer and subduing a man in a fracas at Frome football club.

Taser International, the firm that has supplied 600,000 Tasers to 16,300 law enforcement agencies in 207 countries, predicts revenues of $73m (£46m) from sales of its stun gun range this year. It admits its devices are not risk-free but cites a US study that found 99.75% of 1,201 people suffered mild or no injuries after being hit with an "electronic control device". Taser International claims that more than 80,000 lives have been saved by their deployment. But there is growing disquiet about the way police use what is sometimes billed as a "less lethal" weapon. Amnesty International has recorded 450 deaths after a Taser firing. It is rarely clear whether a Taser has directly contributed towards a death, but in July this year Taser International was ordered to pay $10m (£6m) to the family of Darryl Turner, a teenager who died in 2008 after police shot him with a Taser at a grocery store in North Carolina. A jury found that Taser International failed to warn the police that discharging the weapon into the chest close to the heart risked cardiac arrest. The company also this year lost an appeal against a $6m (£3.7m) wrongful death verdict in the case of Robert Heston, a Californian subjected to 75 seconds of repeated Taser discharges who died of a heart attack.

A rifle that fires bolts of electricity was dreamed up by Tom Swift, the fictional teenage hero of a series of American adventure books for boys in the early 20th century. Jack Cover, a nuclear physicist who worked on the Apollo moon landings, devised the real thing in his garage in the late 1950s and gave it an acronym in honour of Swift (Thomas A Swift's Electric Rifle). Early Tasers used gunpowder to fire electric darts. Classified as firearms, they were not adopted by police departments until the 1990s, when two Arizona brothers, Rick and Thomas Smith, contacted Cover and launched the Air Taser, using the inventor's idea of compressed air rather than gunpowder to propel darts attached to insulated wires bearing an electric charge. The modern electric stun gun was born.

As its use spread across the US, and then around the world, Taser became a verb. "Don't Tase me, bro," shouted Andrew Meyer, an unarmed University of Florida student who was shot with a Taser in 2007 while being wrestled out of a debating hall by police after he had tried to ask a question of former US presidential candidate John Kerry. The YouTube video has been watched 6m times and footage of innocent and not-so-innocent people toppling over like skittles after being shot with a Taser is now an internet staple. The standard Taser in use by police in Britain – and yours for $500 (subject to security checks) from US websites – fires two barbs that can carry wires up to 6.4 metres. These attach to the subject's flesh or clothing, forming a circuit through which an electric current is discharged. Its effect is, victims say, like an agonising cramp. Five seconds later, in theory, they are fine.

Howard Swarray, who was having an epileptic fit when he was Tasered by police. Photograph: Manchester Evening News

The Taser videos are distressing and Swarray cannot bear to watch footage of police restraining him two years ago. His seizure left him unable to remember all the details but eyewitnesses, officials and CCTV from the gym have filled in the blanks. It is common for a person in the recovery phase after a seizure to become aggressive and disorientated, and Swarray sought to escape the gym to cool down, climbing on to the reception desk, at which point he was restrained. A report by the Independent Police Complaints Commission (IPCC) into the incident claimed he "bit, punched and kicked the people who were trying to restrain and treat him", but Swarray's lawyer says no evidence has been produced to support this. Swarray says gym staff told paramedics he was suffering from a seizure but police still struck his arms and legs before firing a Taser at him, handcuffing him and then bending his toes back and standing on his legs to restrain him.

At the hospital, doctors were shocked by his injuries. Since then, Swarray says, he has suffered from depression and panic attacks, as well as nerve damage in his wrists from where he was handcuffed. It is possible that his renal failure was caused by muscle damage triggered by the Taser, according to a consultant cited by the IPCC, although a more likely cause was said to be physical exertion due to the seizure, gym activity or resisting physical restraint. Swarray also bears psychological scars. "When I go out now I'm scared. I hope nothing happens to me and I have a seizure in the street because I may have to go through what I was put through last time – waking up in intensive care," he says. He does not believe police should be using Tasers at all because they can never tell what health problems a person may have. "How are they going to know unless you're walking around with a placard saying 'I suffer from epilepsy' or 'I have a heart condition'? I could be dead."

Satisfied that Tasers posed no risk to the human heart after government scientists fired them into beating hearts extracted from guinea pigs (guinea pigs are considered to have more fragile hearts than human beings; in the US, Tasers have been tested on pigs), the Labour government allowed officers not specially trained in firearms to use them in 2008. This marked a big expansion in their deployment, since they had been trialled only by armed police. Now, every police force in England and Wales has fired Tasers, although in parts of Scotland it is still only armed police who are allowed to carry them. Officers carrying Tasers are given 18 hours' training, and each use of a Taser is supposed to be meticulously recorded. This includes documenting when Tasers are drawn and people are "red-dotted" – that is, the laser sight of the Taser is trained on their chest without firing; this, say police, is an effective deterrent and often enough to ensure compliance.

Peter Kirkham, a former detective chief inspector and now a policing services consultant, says there is an irrational fear of Tasers, which fill a "very definite" gap in policing. "Because they look like guns, people psychologically put them in the same category when they are not. They are certainly not fatal for the vast majority," he says.

According to Kirkham, police find Tasers extremely useful for defusing situations where people are armed with knives or swords, which officers might previously have tackled with guns. "Police officers could not safely deal with that person – they either talked them out of it or firearms were the only option. Tasers fill that gap. People are alive where firearms officers would have shot them in the past." But is there a temptation to overuse it? "Police officers are human," says Kirkham. "If they get a new toy, they like using it."

Amnesty welcomes the use of Tasers in circumstances where they replace guns. "As a weapon of last resort it's clearly better to shoot someone with a Taser than shoot them with a gun," says Oliver Sprague of Amnesty. "But we're concerned they will be used by more people more often in less extreme circumstances. They are being rolled out to more and more officers and the rules on when they are being used are not clear enough." Amnesty is particularly concerned about their use in public order situations; after the summer riots, West Midlands police indicated it would consider using Tasers "more proactively" in riot situations. "As a matter of principle, Tasers should not be used in crowd-control situations," says Sprague. "Officers are likely to miss and hit the wrong person, a Taser is likely to induce panic in a crowd and there's a real danger the officer will be disarmed and then the Taser will be at large."

Guidelines from the Association of Chief Police Officers (Acpo) for police Taser use are "slightly fuzzy", according to Amnesty. The guidelines state that Tasers can only be deployed during "non-firearms situations involving violence or threats of violence of such severity that they would need to use force to protect the public, themselves or the subjects". This is such a wide definition that Tasers could be drawn in all kinds of cases where police would have used traditional methods, such as negotiation or batons. Sprague fears Tasers represent a "slippery slope" in which police become over-reliant on this quick technical fix, which makes them less engaged with – and more likely to alienate – the people they are policing. While "red-dotting" might be a useful deterrent, Sprague points out that threatening to use a Taser is as serious as firing it. "If you can get instant compliance by just pointing a Taser then there's a strong argument for drawing it immediately," he says. Suddenly, policing by consent becomes policing by compliance.

Sophie Khan of GT Stewart, a solicitor who is representing a growing number of victims of Taser use, including Swarray, argues that although Acpo's guidelines advise police to be cautious about Tasering someone small – ie, a child – they do not specifically recommend that officers should not fire Tasers at those suspected of being physically or mentally ill or on medication. "There's a loophole that has to be closed. It's not rocket science to know that you should not Taser someone having an epileptic seizure, but this kind of common sense is missing from police guidance," she says.

"Police officers can only use as much force as is reasonable in the circumstances and officers deploying Taser must justify its use," says Assistant Chief Constable Simon Chesterman, the Acpo lead on Tasers. According to Chesterman, Acpo's guidance is under constant review, with an independent panel of medical advisers monitoring the latest medical studies on the weapon from around the world. "In addition, the Acpo guidance is supported by a detailed training curriculum, which is delivered to all Taser officers and refreshed annually," he says. "We take the learning from IPCC investigations very seriously and their findings and recommendations actively contribute to the development of the guidance and training."

In Britain, the IPCC is currently investigating 33 of 86 complaints this year from individuals shot by police with Tasers. Although the weapon has been declared not to have played a part in two recent deaths in Britain, the IPCC is still investigating the case of Dale Burns, a 27-year-old taxi driver from Barrow-in-Furness who became embroiled in a physical confrontation with police and was, friends claim, shot with a Taser four times. Burns, a fit young bodybuilding enthusiast with no history of heart trouble, died in hospital later that evening. A pathologist's report is expected imminently. Helen Shaw of the charity Inquest says whatever the pathologist's findings the medical evidence in Burns's case should be tested in court before the Taser is hastily acquitted of any involvement in his death.

Earlier this year, the IPCC concluded its investigation into the use of the Taser against Swarray. "With hindsight actions such as giving commands and attempting compliance through pain to a person who was already known to be unresponsive were questionable," said IPCC commissioner Naseem Malik in her report, which called on Acpo to "consider" whether "less violent tactics" could be used in medical emergencies. Nevertheless, the IPCC concluded that officers "acted correctly" and nothing in Acpo or Greater Manchester police guidelines prevented the use of a Taser against a person who is having an epileptic seizure. Although Malik commented on the "inappropriate language" of one officer that "suggested a certain mindset" she judged the officer had "considered tactical options before deciding to use the Taser".

Swarray is dismayed that the IPCC investigation did not censure the police and is to launch a civil action against Greater Manchester police. He believes that, since its verdict, the use of Tasers has increased. "It's ironic that since my case they've been using Tasers with more vigour and brutality. If they can get away with doing it to someone who is ill there's no hope for anybody," he says. "It's scary. It's been really traumatic and difficult to deal with."