Howman also believes 10 percent of elite athletes could still be doping as WADA continues its efforts to weed out drugs cheats and those that supply them with illegal substances.
Speaking to the BBC, Howman said: "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.
"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 a way to make a lot of money."
While elite athletes are routinely tested, Howman says teenagers competing in junior events are "under the radar".
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Howman, who will step down in 2016 after 12 years at WADA, said research suggests that despite improvements in some sports, namely cycling, doping is still too prevalent.
"I have great respect for the way in which the UCI (world cycling's governing body) are now running their anti-doping programme," he told BBC's Harttalk.
"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.
"Not in all sports, in some sports."
Howman said several top athletes were being investigated.
"Not to the same level as the information we had on Lance Armstrong but certainly we have information that is being investigated about a number of athletes around the world," he said.
(Reporting by Martyn Herman; editing by Sudipto Ganguly)
