The head of US Anti-Doping Travis Tygart fears some athletes will "take advantage" of reduced drug testing during the coronavirus pandemic.
Tygart says the outbreak, which has led to almost 24,000 deaths globally, opens a “window of opportunity” for those not "willing to compete clean”.
US Anti-Doping (Usada) is continuing to carry out “mission critical testing”.
However, Tygart thinks the postponed Tokyo 2020 Olympics could be the “cleanest Games we’ve seen”.
"People around the world have indicated significant reductions [in testing], in some places it may have shut down entirely," the American told BBC Sport.
"I think clean athletes are going to compete clean and are not going to try and take advantage of this situation.
"What we worry about is those who otherwise may not be willing to compete clean and will try everything to get away with it. It does for a period of time at least open that window of opportunity.
"We know that there are those who are going to try to exploit every opportunity they can. Certainly it’s a concern and it’s something the world has to take seriously."
On Tuesday, the Tokyo 2020 Olympics and Paralympics were postponed until 2021 because of the Covid-19 pandemic.
Tygart says the decision to put the Games back came as a big relief and has “taken the pressure off”.
"For athletes this was a terrible situation - for training, team selection and in addition the anti-doping concerns it brought," he said.
"It’s not fair not to have full global programmes operational at their maximum capacity six months from the Games. That's the most critical time for it.
"It’s taken the pressure off immensely, and I think we have a wonderful chance to make these the cleanest Games we’ve ever seen."
'We will return the system to full power after this pandemic'
Witold Banka, the president of the World Anti-Doping Agency (Wada), agrees with Tygart's assessment that postponing the Olympics is good for anti-doping and issued a warning for athletes who might want to take advantage of the current reduction in testing.
"This is a really challenging time but anti-doping never sleeps," Banka told BBC Sport.
"Of course this a difficult situation for our environment but we need to do everything to protect the integrity and the unity of anti-doping and make a lot of efforts to do it.
"In some cases testing will be temporarily suspended or limited or cancelled for some time but we have other tools, the testing is not only one weapon.
"This is my strong message to everyone who wants to cheat us: we will catch you. It's not a good space and a good situation for cheats, we will do everything to protect and maintain the unity of the system and I'm sure we will return the system to full power after this pandemic and we will be stronger."
Banka, a former athlete who was Poland's minister of sport before replacing Sir Craig Reedie as Wada president at the start of 2020, also said he was against major events going ahead in the current climate, after the French sports minister suggested this year's Tour de France could take place without spectators.
"I think we should cancel the major events, and we should do everything to protect the safety of the athletes, the fans, the media and everybody who want to participate and watch this race," he said.
"[The pandemic is] really a huge problem and we really need everything to handle this problem and not to think about races, about major events."