Doping is so prominent within the Tour de France that 80% of finalists between 1996 and 2010 either failed drug tests or confessed to doping. Glucocorticoids are commonly used performance enhancers as they are effective anti-inflammatories and pain suppressors and enable a rider to lose weight while maintaining performance. While these drugs are banned in-competition, controversially, they are allowed outside of competition periods as they are believed to have short term effects.

Source: one, two, three, four