Jamaican sprinter Usain Bolt has revealed that he has to stay "constantly vigilant" to remain drug free.
Athletics has been blighted by doping scandals in recent months after athletes, including the former 100m world record holder Asafa Powell, tested positive for banned substances.
The multiple Olympic champion says he makes sure he works with people he trusts as ITV News' Paul Davies reports:
The world's fastest man and Manchester United fan Usain Bolt has told ITV News that he is sure David Moyes "heard" his recent Instagram video message in which the sprinter said the Old Trafford boss needed to invest in his midfield.
"I was sitting at home and watching the game (against Liverpool) and I saw that in the midfield is where we are most weakest so I was just voicing my concern.
"I think David Moyes heard, I'm sure, because it was all over the news. I think he surely got that message. He got (Marouane) Fellaini (in) and (Shinji) Kagawa is now playing so the team is much better and a lot more stable."
Six-time Olympic champion Usain Bolt is reconsidering his retirement after the Rio 2016 Olympic Games and says he is likely to compete for a further year after that.
In an interview with Channel 5 News at his autobiography book launch at London's Selfridges store, he said: "I am definitely reconsidering. I think my fans especially have really voiced their concern about me retiring.
"They think I should carry on and so do my sponsors. I have discussed it with my coach and he says it is possible. We will see what happens but it's on the cards that I will extend it by one more year."
Hundreds of fans have queued in London to catch a glimpse of the world's fastest man, Usain Bolt, as he signs copies of his new book 'Faster Than Lightning' in Selfridges.
At the launch of his new autobiography 'Faster Than Lightning' in London, the world's fastest man said: "If you want to be the best, or you want to strive for more, you've got to set goals in life."
British sprinter James Ellington has described winning bronze in the 4x100m relay before being disqualified as the "best and worst day" of his career.
Best and worst day of my athletics career so far all within an hour,we deserved better.
Thanks everyone for the support,we will be back and even better
British sprinter Adam Gelili has tweeted about his team being disqualified from the 4x100m relay at the World Athletic Championships in Moscow today:
Thank you all for the messages of encouragement. To be so close to receieving a medal and have it taken away is heartbreaking
As a team we will pick ourselves up and carry on until we get it right. Onwards and upwards!
BIg up to the GB 4x100m women on receiving the news for bronze. Extremely proud! Just a shame they don't get their moment on the podium
Great Britain has lost its bronze medal to Canada after being disqualified over the second changeover in the 4x100m relay.
Dwain Chambers crossed the line third but the change between Harry Aikines-Aryeetey and James Ellington took place outside the designated box.
Canada was then awarded third place on appeal.
"It's heart-breaking," Aikines-Aryeetey said. "We found out when we were walking to the medal presentation."
Team GB loses its bronze medal in the 4x100m men's final for an illegal changeover.
Usain Bolt and the Jamaica relay team won the 4x100m gold in Moscow, with the 100m champion finishing ahead of US rival Justin Gatlin in the final leg.