It’s not easy to get hold of Ángel Hernández, the legendary Mexican chemist who for a decade provided illicit performance-enhancing drugs to numerous athletes, including, he claims, all eight 100 metres finalists at the Beijing Olympics. It took me just over a year of trying. The FBI also struggled. The story goes that when they eventually caught up with him in 2005 he had been holed up in a hotel room in Texas, living under an assumed name for two years. Presented with numerous incriminating wire-tapped telephone conversations and bank statements as part of the investigation that eventually sent three-time Olympic gold medallist Marion Jones to prison, Hernández became a state witness in return for avoiding chokey.

Today, he says, he is clean, no longer juicing athletes but instead working in top-level boxing (‘It’s terribly dirty’), putting his deep knowledge of sports science to use as a conditioning expert. ‘I’ve been clean since I became a Federal witness and that is the only way for me until the day I die.’

Perhaps more than anyone else alive, Hernández — who was previously known as Ángel ‘Memo’ Heredia — understands the charade that top-level international sport has become and the inability of drug-testing programmes to prevent athletes from cheating in order to win enormous sums of money. He knows what athletes — ‘like the soccer players in London’ — are prepared to invest in order to ensure they are quicker and fitter than the other guy.

Are footballers really doping? ‘Yes, of course.’ He says drug testing in the game is nowhere near frequent or aggressive enough to prevent it. ‘Think about it; if you are a soccer player you can do a blood transfusion [to boost oxygen levels in the blood]. If there is no testing, like a biological passport, then you can get away with a blood transfusion. Get away with it easily. You can still get away with all the other tricks in the house, too. You can still get away with micro--dosing EPO [a red blood cell booster], micro--dosing IGF-1 [a muscle growth hormone].

‘The problem is that the testing has got better, but it has not got to the point where they can detect everything, as they say they can.’ He adds that drug-testing bodies are reluctant to admit publicly the limitations of their testing procedures for fear of losing funding. ‘They have to justify to the public how much money they make, saying they are working on detection methods, when in reality the detection methods they have are not 100 per cent reliable.’

Hernández says a major problem for testers is the ever-increasing number of drugs coming on to the market. ‘You have got to understand that as the pharmaceutical industry grows year by year, new drugs [with performance-enhancing qualities] come to the public or into the research environment. What happens is the testers don’t catch up until later on. There is always a gap. Some of the athletes are cheating now; they are taking advantage of an opportunity that lasts a year, maybe a year and a half, before the testing authorities have any idea what is happening. They are abusing those drugs.’

Drug-testing bodies are today supposed to hold athletes’ samples for a minimum of ten years so they can be retested in future, when new ones might have been developed. However, in reality that practice is not always observed. Hernández says that he knows of testing bodies that dispose of samples almost immediately.

‘If you don’t hold those samples for the ten-year rule, then there is nothing you can do. You can assume, but you have no proof,’ he says.

On the subject of drugs that are easily available over the counter in any pharmacy and that have performance-enhancing qualities, Hernández is fascinating, although he refuses to name the most potent ones for fear of facilitating cheating. He will say Viagra is a drug that can make a sprinter run faster, but that it’s not on the banned list of athlete medications. ‘If I was a sprinter, I would not take Viagra,’ he says. ‘You do not want to have a boner while you’re running a race.’

What about asthma inhalers? So often we see top-level endurance athletes sucking on inhalers, a sight at odds with most people’s childhood memories of asthmatic classmates’ loathing of aerobic exercise. Could it be the humble asthma inhaler holds performance-enhancing qualities?

‘I call it the “transporter”,’ he says. ‘It opens and expands not only your lung capacity but also your pulmonary capability, so it has improved capacity to move the blood cells…

‘In other words, if you were using EPO, or if you were using another substance like EPO, it would help you to boost endurance even more. It is like multiplying the effects by between three and five times.’

He points out some athletes are using the pumps legitimately because they have what he calls ‘induced asthma’ from training, but that others are cheating by conning doctors into giving them medical letters stating that they have the condition, letters that no anti-doping agency on the planet can argue with. ‘It’s like a green light for doping,’ he says.

Certainly, this is a version of events that tallies with the experience of elite American distance runner Lauren Fleshman, who in 2015 spoke publicly of the pressure she felt she was under to be diagnosed with asthma.

She claims that coach Alberto Salazar — head of the Nike Oregon Project — ‘set up an appointment in Portland, during allergy season, with a doctor who had seen many other runners. He had a specific protocol… you would go to the local track and run around it, work yourself up to having an asthma attack and then run down the street, up 12 flights of stairs to the office and they would be waiting to test you. So that is what I did and I failed the test.’

Once prescribed Advair, to be taken by inhaler, she claimed Salazar ‘encouraged me to push to be on the highest dose of it year round, which was something different than what the doctor said’. Salazar denies the allegations.

Did Hernández encourage the athletes he doped to use asthma pumps? ‘Of course,’ comes the reply. ‘All of my athletes were using pumps.’ I ask him if at the elite level it is possible for a clean athlete to beat an athlete who is doping. He says in boxing it would be very difficult, because the doped fighter would be able to throw heavy power punches from round one to 12 without tiring. How about in sprinting? He is emphatic. ‘As a sprinter you cannot, no. No.’

Hernández stresses that he is a world expert in human fitness and conditioning, now walking on the right side of the law. He expresses frustration that his past still clings to him, and that if a boxer he is training wins a fight, aspersions are immediately cast.

‘I’m being judged every time my athletes get a good performance, because of science and training. But the problem is not me. The problem is the system itself. It allows the athletes to have the tools and the abilities to cheat.’

I finish by asking if he would want his son to be an athlete. A note of resignation comes into his voice: ‘When I was young I was a national champion, a record holder for the discus, Central America champion, you name it. I was projected to become an Olympian. But then, after seeing so many things, that’s when I started understanding what the whole business is about. It changes your view. I want my son to be what he wants to be. An athlete, a pianist. But frankly, I don’t want him going into the Olympic world, or boxing. A scientist or a doctor, maybe. Sports are getting worse and worse.’

Damian Reilly and the Guardian's Sean Ingle discuss the doping epidemic on the Spectator Podcast.

The testers don't catch up till later on. There is always a gap. Some of the athletes are cheating now

Spectator.co.uk/podcast Damian Reilly and Sean Ingle on doping in sports.