London Marathon elite runners face more drug tests, as a former winner was stripped of her title for doping.
Russian Liliya Shobukhova, who won in 2010 and was runner-up in 2011, has had her results since 2009 annulled.
London Marathon chief executive Nick Bitel said athletics' world governing body, the IAAF, had agreed to increase out-of-competition tests for athletes at the world's six major marathons.
"We feel that is the best way of ensuring our sport is clean," he said.
"We've just agreed with the IAAF that we're going to carry out hundreds and hundreds of out-of-competition tests on athletes ourselves," he told BBC Radio 5 live.
"We're not just sitting there waiting for somebody else to find out the results, we're being very proactive about this."
Abbott World Marathon Majors, the umbrella group for the six major marathons of London, Tokyo, Boston, Berlin, New York and Chicago, is funding the increased number of tests for athletes taking part in its events.
Out-of-competition testing is being widened as of this summer, with AWMM paying for up to an additional 900 tests to be carried out.
Shobukhova, 37, was initially banned for two years in April 2014 after irregularities were detected in her biological passport.
Following a lengthy appeals process, the 2006 European Championships silver medallist has now had all her results annulled from October 2009.
"She was caught some time ago and her appeals process has taken nearly two years and that really is a great deal of time and it's something that we're not particularly happy with," said Bitel.
London Marathon organisers will now be taking court action "to recover the prize and appearance money" paid to Shobukhova in 2010 and 2011.
Aselefech Mergia is now elevated to the winner's spot in the 2010 race, with fellow Ethiopians Bezunesh Bekele second and Askale Tafa third.
Meanwhile, Kenya's Edna Kiplagat takes second place in 2011, with Bekele third.