For many years the Tour de France has been embroiled in allegations of riders using banned substances to enhance their performance.
Long-running allegations against American Lance Armstrong were never proved but riders are routinely tested and teams have found themselves raided by the police during events.
Current Tour leader Michael Rasmussen is under suspicion after allegedly failing to inform drugs testers of his whereabouts on four separate occasions.
He has denied any wrongdoing and continues in the event.
Here PA Sport looks at some of the other riders caught up in drugs problems.
Alexandre Vinokourov was yesterday withdrawn from the Tour by his team after it was revealed he had tested positive for a banned blood transfusion. He has protested his innocence and asked for his B sample to be tested.
Floyd Landis won the 2006 Tour but has since been embroiled in drugs allegations and has been fighting to clear his name after it was announced he had failed a drugs test.
The ongoing Operation Puerto has been conducted by authorities in Spain against doctor Eufemiano Fuentes. Since May 2006 a number of cyclists, tennis stars and football players have been linked to the investigation. Top cycling stars Ivan Basso and Jan Ullrich missed the 2006 Tour because of the case.
British cyclist David Millar was banned from the sport in 2004 for taking EPO but has since returned to the sport and is taking part in the current Tour de France.
In 1998 the Festina-backed team were caught up in a doping scandal on the Tour. Several members of the team staff were arrested and convicted of trafficking and giving doping substances.
Italian Marco Pantani won the Tour and the Giro d'Italia in 1998 but failed a blood test during the Giro the following year. He died of a cocaine overdose in 2004.
British cyclist Tommy Simpson died on the Tour in 1967 after collapsing twice on a steep climb. A post mortem revealed traces of amphetamine and methylamphetamine in his blood, but the amounts were not thought to be large enough to have contributed to his death.
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