The New Zealander who leads the World Anti-Doping Agency estimates more than 10 percent of elite athletes could be using performance-enhancing drugs.
The agency's director-general, David Howman, said the number of children doping to reach the elite level was the biggest concern.
Mr Howman said the increasing profitability of sport had led to young athletes becoming more vulnerable. He said they were usually under the radar of doping agencies, as drug testing was limited in non-elite events.
He also said doping could become a criminal offence within five years.
Speaking on the BBC's Hardtalk programme, Mr Howman said far fewer athletes were being caught than were believed to be using drugs.
"We have some guestimates based on some research undertaken over the last years.
"It's far more than we would wish it to be - over 10 percent. That is of concern because those being caught by the system is far lower than that."
He said it was not occurring in all sports.
"The area of most concern for us is the level of young athletes who have not broken through into the elite who are trying to get that breakthrough and are susceptible to taking drugs because that's a shortcut," he added.
"Not only are they susceptible to taking drugs, they are being encouraged to do so by any one of a number of people that surround them - coaches, trainers, even parents - because it's way to make a lot of money."
Nigerian weightlifter Chika Amalaha, 16, was stripped of her gold medal after failing a drugs test at last year's Commonwealth Games.
Former World Anti-Doping Agency president Dick Pound has previously estimated that four out of five cheats were not being caught.
Mr Howman said he wondered whether there were still riders doping on this year's Tour de France after former winner Lance Armstrong was stripped of seven titles following his admission he took performance-enhancing drugs.