It was launched as a wonder drug. A tablet to take the pain out of kicking the killer weed.
Since it appeared on the market two years ago in the UK, almost 400,000 prescriptions for Champix have been written.
Across the world, that figure is currently sitting at nine million.
Champix, or varenicline, to give it its clinical name, was first licensed here in December 2006.
It mimics the effects of nicotine on the body so it both reduces the urge to smoke and relieves withdrawal symptoms.
Robert McGhee found his wife Karen hanging from the hall stairs
In clinical trials, it proved more effective than alternative remedies at getting people to quit.
For its makers - Pfizer - Champix has been a huge success.
In its first full year on the market, the drug brought in a staggering $883m for the company.
But about a year ago, I became aware of stories emerging in the media in which people who had taken Champix were said to have suffered severe depression.
I learned of a woman, Karen McGhee, from Greenock, who tried to kill herself just a few days after taking Champix.
Her husband Robert told me of the night he found his wife hanging from the hall stairs.
Robert McGhee said: "I just got woken up with my daughter dragging me out of my bed to come downstairs. She was shouting, 'Dad, my mum's hung herself. She's hung herself'. And I just couldn't comprehend it, I thought I was dreaming.
"I ran downstairs and Karen was on the floor on her knees with a pelmet round her neck. I tried to lift her up to free the knot, but I couldn't get it loosened. I managed to cut her free and take the noose off her neck."
After several days on a ventilator, Karen McGhee pulled through.
She is convinced it was Champix which led to her suicide attempt.
She said: "I'll never put myself through that again and never put my family through it again. I'll always be a smoker, or, if I quit, then I'll quit naturally."
I heard of another case - Omer Jama - a young Manchester man who had killed himself while taking Champix.
His brother, Ali, said: "He'd slashed his wrists. They'd found him on the floor in the landing and it was a shocking scene. There was no suicide note. It was just a random act, completely out of character, that took an instant."
Omer had clearly taken his own life. But surprisingly, at the inquest, the coroner did not record his death as suicide.
Ali said: "The coroner recorded an open verdict because she couldn't record a verdict of suicide, because on the evidence of the forensic toxicologist she found that Champix was still in Omer's bloodstream and she had researched that it had possible links to suicidal tendencies. So she couldn't rule out that it didn't play a part in his suicide."
The company warns of side affects with the drug
I discovered that across Britain, more than 3,000 people have complained about adverse reactions to Champix.
About 260 have reported suicidal-related reactions to the drug. Of these, 16 had attempted suicide and 10 had killed themselves.
But 260 reported bad reactions, no matter how severe, sounds like a drop in the ocean compared to millions of Champix prescriptions. I wanted to know whether these extreme reactions were just isolated cases.
I went to the US where the drug - marketed there as Chantix - was developed and first launched.
I met James Elliott, a veteran of the war in Iraq, who had been given the drug by the US Government's department of Veterans Affairs, as part of his treatment for post-traumatic stress disorder.
One night, a few days into the Chantix course, he fought with his girlfriend, grabbed a handgun, walked into the Washington DC night, and forced the police into an armed stand-off.
James told me: "I think there were nine officers there. And two of them had rifles trained on my head. Those guys would not have missed. I antagonised them. You guys got guns. You tough guys. Shoot me. It'll be alright."
The police used a taser gun to subdue him, and he's lucky to be alive today.
His story prompted an investigation by the US Congress.
Giving evidence, he told them he was never warned about the possible side effects.
He is adamant Chantix played a part in his actions that night.
He said: "I put the whole community at risk. I did. The doctors did. Chantix did."
During my investigation, I also met one of the country's leading personal injury lawyers.
Marc Grossman has launched five law suits against Pfizer and has 20 more cases ready to go.
He said: "We have a 40-year-old man with three children who has no prior history of psychiatric problems who, out of the blue, went and put a shotgun to his mouth and killed himself.
"And another case is of a man who shot himself in the head.
"We have a woman with kids who hung herself.
"All the cases are just the most shocking, bizarre scenarios where someone just out of the blue commits suicide without any prior depression."
I suggested to him that it wasn't unusual for people to commit suicide out of the blue with no history of depression.
The pills are made by drug company Pfizer
He told me: "It doesn't happen as often as with the people taking Champix. It happens far more often that you'd ever expect and far more often than is acceptable."
Champix is big business, but it is also designed to be the answer to one of the most serious health issues facing the world, and especially Scotland.
Here, 13,000 people die every year from smoking-related illnesses, and it's the single biggest preventable cause of death.
Against that backdrop a drug which stops people smoking would be welcomed, and even a small decrease in smokers would loosen up resources across the Scottish health service.
Pfizer declined to speak to the programme, but a statement from the company said the packs carry warnings about the reported side effects, and point out that there's no proof the drug has any links with suicidal behaviour.
It said: "Pfizer rigorously tested varenicline in a comprehensive clinical trial programme lasting many years and involving thousands of patients.
"Based on the results of those clinical trials, regulatory agencies in Europe, the US and elsewhere have recognised the safety and efficacy profile of varenicline and approved the medicine for use in smoking cessation in adults, as described in the labelling information for the product.
"As indicated in the labelling information for varenicline, stopping smoking, with or without treatment, is associated with nicotine withdrawal symptoms and the exacerbation of underlying psychiatric illness.
"Depressed mood may be a symptom of nicotine withdrawal. Depression, rarely including suicidal thoughts and suicide attempt, has been reported in patients undergoing a smoking cessation attempt, including patients taking varenicline.
"If any of these symptoms are experienced and are of concern, treatment should be stopped immediately."
The body which licenses pharmaceuticals for use across the whole of the European Union tells us it's monitoring the drug closely, but at the moment feels the benefits of Champix far outweigh any current potential risks.
Dr Hans-Georg Eichler, senior medical officer with the European Medicines Agency (EMEA), told me: "This drug is designed and has been shown to be effective in helping patients quit smoking.
"Now we all know that there are several hundred thousand premature deaths across the European Union every year.
"This is a major public health issue. We know that a large proportion of smokers want to quit but they find it difficult.
"So against that background any drug, and I emphasise any drug, that has a positive benefit risk profile is valuable and welcome.
"I think the public is justified to be concerned and we're extremely concerned.
"But being concerned is not the same as being convinced that there is a causal relationship.
"So that's why we have implemented a high alert monitoring programme.
"We have here in the agency weekly meetings where we would analyse all the incoming data, and would alert the committee that is relevant, the working party, that is charged to specifically look at these events and see what action is required."
Ashes to Ashes: Is Champix safe? Was broadcast on Wednesday 26 November on BBC One Scotland.