How do drugs like human growth hormone and EPO make their way into professional sport?
STEP ONE: In the lab
The process begins in scientists' laboratories, where legitimate research is going on into products that could be used to treat existing medical conditions.
Michelle Verroken, director of drug-free sport at UK Sport, takes up the story.
"It's not that surreptitious," she says. "Scientists will publish their work in journals for review, because they want other scientists to critique their work. So it's all out there for us to look at."
Those sportsmen or coaches looking for an extra edge will study these latest scientific findings in the hope of finding a new product or treatment that could also have an application for them.
STEP TWO: The rumour-mill
As John Brewer, head of human performance at the National Sports Centre in Lilleshall explains, the process then picks up pace.
"The one thing you will always find with elite athletes is that they will always look for something to give them an edge," he says.
"Hopefully, in the vast majority of cases, that will be legal - different training, or a different diet. But people will seize on something, even if there is no proven research that suggests it actually has a genuine benefit.
"There are a lot of substances tried that might not work. But even if there is a half-report or suggestion that a drug might have a particular effect in a particular case, there will be coaches and athletes there who will advocate trying it to see if it works.
"Rumours spread like wildfire. They'll pick up in the changing-room that someone has tried something that works, and then for fear of being left behind, they'll try it to see if it works for them too."
STEP THREE: From internet to changing-room
Once an athlete or coach has heard about a drug and decided they want to try it, how do they get hold of it?
"The internet has opened it up totally," says Brewer. "Whereas 10 years ago it was very difficult, now you can type anything in a search engine, and you'll find someone out there keen to help.
"There's a huge market out there. I'm not sure you can even blame the big drugs companies - it's often back-street people mixing up their own concoctions."
"The internet has caused huge problems," confirms Verroken, "because where the licensing laws in one country would prevent the sale of something like EPO over the counter to athletes, that might not be the case in other countries.
"A black market emerges and takes some of those substances and recycles them back in to other parts of the world.
"We've seen pharmacists sell off substances. We've seen drugs disappear from the supply chain, and there have been very few drug companies who have responded with major investigations.
"Right at the very end of the supply chain you get individuals who need the drugs - but they need the money more, so they sell off. It's an on-going problem."
CASE STUDY: EPO
The story of how EPO came into sport is an informative example.
Erythropoietin is a naturally occurring hormone, produced by the kidneys, which stimulates the body to produce more red blood cells.
American pharmaceutical company Amgen were the first to manufacture a recombinant (synthetic) version of EPO, which they called Epogen.
It began to be used widely - and effectively - in medicine in the late 1980s as part of the treatment of anaemia, a condition caused by a low number of red blood cells.
Sportsmen soon realised that EPO would also boost the number of red blood cells in a healthy person's body - meaning their blood could carry more oxygen, so their bodies could work harder for longer.
It was picked up by athletes in endurance sports - cycling, distance running, swimming - and by the late 1990s had become the key battle in the anti-doping war.